Teacher Needed?

You need a teacher for what? If it is information, they are all found in books. If realisation, how can anyone make you realise anything?

As far as the Nikayas go, it is not a teacher (acariya) one needs, but good friendship (kalyanamittata).

"And what is good friendship? Here, in whatever village or town a clansman lives, he associates with householders or their sons—whether young but of mature virtue, or old and of mature virtue—who are accomplished in faith, virtuous behavior, generosity, and wisdom; he converses with them and engages in discussions with them. Insofar as they are accomplished in faith, he emulates them with respect to their accomplishment in faith; insofar as they are accomplished in virtuous behavior, he emulates them with respect to their accomplishment in virtuous behavior; insofar as they are accomplished in generosity, he emulates them with respect to their accomplishment in generosity; insofar as they are accomplished in wisdom, he emulates them with respect to their accomplishment in wisdom. This is called good friendship."

(AN 8.54, tr Bhikkhu Bodhi, tr Narada Thera; see also: Association with the Wise)

As for the Mahayana:

"Bodhisattvas on the beginning level, intent on practising prajna

To seek unsurpassed bodhi, get close to good and wise friends.

How to obtain great wisdom and merit? Should be from prajnaparamita.

That is how all buddhadharma and merit are attained from good friends."

(Ratnagunasamcayagatha 15.1-2, tr from T229p680a28-b2)

"The mind not being intimidated and such,

Those who teach the lack of nature and so on,

And abandoning the antagonistic factors of these

Means being mentored in every way."

(Abhisamayalamkara 1.36, tr Brunnhölzl)

Both the Astasahasrika (PP8K) and the AA commentaries explain that the good friend is the one who teaches prajnaparamita, in particular that all appearances are empty, while the bad friends are those who teach hinayana. See: PP8K 1.2, 15.1, 22.1, 30.1; and Gone Beyond, vol 1, p 282-283;  Groundless Paths, p 123-124, 422.

The Diamond Sutra gives the following summary of who teaches what:

"The dharmas spoken by the Tathagata cannot be grasped and cannot be spoken. They are neither dharmas nor no dharmas. And why? Unconditioned dharmas distinguish worthy sages."

and

"all Buddhas and all Buddhas’ Dharma of Anuttarasamyaksambodhi come forth from this Sutra. Subhuti, the Buddhadharmas spoken are no Buddhadharmas."

(ch 7 and 8, in Diamond Sutra, p 102, 106)

Huineng explains that like this:

"Good friends, if you wish to enter into the profound dharmadhatu and the samadhi of prajna, you must cultivate the practice of prajna and recite the Diamond Sutra. Thus will you attain seeing the nature. You should realize that the merits of this sutra are immeasurable and unlimited. They are clearly praised within the sutra; I cannot explain them fully here. This teaching is the Supreme Vehicle: it is preached for those of great wisdom, it is preached for those of superior capacities. Those of small capacities and small wisdom who hear it will generate doubt."

(Platform Sutra, ch 2, BDK ed, p 31)

He also says,

"You should each contemplate your minds and each see the fundamental nature. If you do not become enlightened by yourself, then you must seek a great spiritual compatriot, someone who understands the Dharma of the Supreme Vehicle, to indicate directly the correct path for you. This spiritual compatriot will have a great background and will, so to speak, lead you to the attainment of seeing the nature. This is because the spiritual compatriot is able to manifest the causes of all the good dharmas. All the buddhas of the three periods of time and the twelve divisions of the canon are fundamentally and naturally immanent within the natures of people, but if you cannot become enlightened yourself, you must seek a spiritual compatriot’s instructions in order to see [the nature].

If you can become enlightened yourself, don’t rely on external seeking — don’t think I’m saying you can only attain emancipation through [the help of] a spiritual compatriot other than yourself. This is not the case! Why? Within your own minds there is a spiritual compatriot [who will help you] become enlightened by yourself! If you activate the false and deluded, you will become all mixed up with false thoughts. Although some external spiritual compatriots may be teachers, they cannot save you. If you activate the correct and true and contemplate with prajna, in a single instant [all your] false thoughts will be completely eradicated. If you recognize the self-nature, with a single [experience of] enlightenment you will attain the stage of buddhahood."

(Platform Sutra, ch 2, BDK ed, p 33)

There are people who believe that a teacher is needed to point to the nature of mind and transmit the Dharma. However, the nature of mind cannot be shown, nor is there a Dharma that could be transmitted.

"In my view there is no Buddha, no sentient beings, no past, no present. Anything attained was already attained—no time is needed. There is nothing to practice, nothing to realize, nothing to gain, nothing to lose. Throughout all time there is no other dharma than this. ‘If one claims there’s a dharma surpassing this, I say that it’s like a dream, like a phantasm.’ This is all I have to teach."

(Record of Linji, p 12-13, tr Sasaki)

What do teachers give then?

Yaoshan hadn't been to the lecture hall for a long time. 

The temple supervisor said, "We've all been looking forward to your giving us a lecture." 

Yasohan said, "Ring the bell." 

As soon as the monks had gathered for the lecture, Yaoshan got up from his seat and went back to the abbot's quarters. The supervisor followed and asked why he didn't say anything, since he had agreed to speak to the monks. 

Yaoshan said, "They have teachers to teach them sutras and they have teachers to teach Abidhamma, so what is there left for me to do?"

(Record of Yaoshan, in Soto Zen Ancestors in China, p 63)

So Yunmen made it clear:

Having entered the Dharma Hall for a formal instruction, the Master said:

“All of you who come and go for no reason: What are you looking for in [this monastery] here? I only know h ow to eat and drink and shit. What else would I be good for?

“You’re making pilgrimages all over the place, studying Chan and asking about the Dao. Let me ask you: What have you managed to learn in all those places? Try presenting that!”

Again, he said: “In the meantime, you cheat the Master in your own house. Is that all right? When you manage to find a little slime on my ass, you lick it off, take it to be your own self, and say: ‘I understand Chan, I understand the Dao!’ Even if you manage to read the whole Buddhist canon— so what?!”

(Record of Yunmen, p 154, tr App)

And Huanglong explained further:

Huanglong addressed the monks, saying, “Before I came up here to speak there was nothing in my mind. But now that I’ve come up here there are a lot of questions. I dare to ask you whether the great vehicle of our school is found in such questions and answers. If it were to be found in such speech, then doesn’t the scriptural canon have questions and answers? Yet it is said that [the way of Zen] is transmitted outside of the scriptural teachings. It is transmitted to individuals who are great Dharma vessels. If it can’t be found in words, then even if you ask all sorts of excellent questions, what, after all, is the point of doing so? ... If you want to talk about it, then you can say that it can’t be realized through mystical perception or self-perfection. Nor may it be said to be a result of some all-encompassing understanding. The buddhas of the three worlds have only said you must know yourself. In the entire canon of scripture this can’t be explained. ... Those who leave home must have heroic resolve, cut off the two heads, and practice in seclusion in the house of the self. Afterward they must throw open the door, get rid of the possessions of that self, and then receive and meet whatever comes, giving aid to any in need. In this way the deep compassion of Buddha can be in some small measure repaid. Aside from acting in this manner, there is nothing else.”

Huanglong then struck the meditation platform with his whisk and left the hall.

(Zen's Chinese Heritage, p 403-404)

The story of Dongshan is a good example:

Once, when Yün-yen was making some straw sandals,  Tung-shan approached him and said, "I would like to have the Master's eyes."

Yün-yen said, "Where have yours gone?"

"Liang-chieh has never had them," replied Tung-shan.

Yün-yen said, "Supposing you did have them, where would you put them?"

Tung-shan said nothing. Yün-yen said, "Isn't it the eye that desires eyes?"

"It is not my eye," replied Tung-shan.

"Get out!" thundered Yün-yen.

(Record of Dongshan, p 26-27, tr Powell)

And later when asked about Yunyan:

Because the Master was conducting a memorial feast for Yün-yen, a monk asked, "What teaching did you receive while you were at Yün-yen's place?"

The Master said, "Although I was there, I didn't receive any teaching."

"Since you didn't actually receive any teaching, why are you conducting this memorial?" asked the monk.

"Why should I turn my back on him?" replied the Master.

"If you began by meeting Nan-ch'üan, why do you now conduct a memorial feast for Yün-yen?" asked the monk.

"It is not my former master's virtue or Buddha Dharma that I esteem, only that he did not make exhaustive explanations for me," replied the Master.

(p 28)