Gemara

Rebbi: I am sharper than my colleagues because I saw R. Meir from the back. Had I seen him from the front, I would be even sharper - "v'Hayu Einecha Ro'os Es Morecha."

54b (Beraisa): In the Midbar, Hash-m taught Moshe. Aharon entered, and Moshe taught him. Aharon went to sit to the left of Moshe. His sons entered and Moshe taught them;

Elazar sat to Moshe's right, and Itamar sat to Aharon's left;

The elders entered and Moshe taught them.

Kerisus 6a - Rav Mesharshiya (to his sons): Before learning from your Rebbi, learn the text [by heart]. When he teaches, look at his mouth - "v'Hayu Einecha Ro'os Es Morecha."

Megilah 21a (Mishnah): One may read the Megilah standing or sitting.

(Beraisa): This does not apply to Torah. (One must stand while reading it b'Tzibur.)

(R. Avahu): We learn from "stand here with Me." It was as if also Hash-m stood!

This also teaches that the Rebbi should not sit on a bed while teaching Talmidim sitting on the ground.

(Beraisa): From the days of Moshe until R. Gamliel, people stood while learning Torah. After R. Gamliel died, people became weak, and learned while sitting;

(Mishnah): The honor of Torah ceased when R. Gamliel died.

Contradiction: It says "va'Eshev (I sat) on the mountain", and it says "I stood on the mountain"!

Resolution #1 (Rav): He stands and learns, and sits and is Shoneh (reviews his learning).

Resolution #2 (Rava): He learned gentle matters while standing, and harsh matters while sitting.

Bava Metzi'a 84b: Rebbi and R. Elazar b'Rebbi Shimon used to sit on the ground in front of Rabanan, who were on benches.

Sanhedrin 17b (Rav Yehudah): In Yavneh, there were four who could speak [every language] - R. Eliezer, R. Yehoshua, R. Akiva and Shimon ha'Temani, a Talmid who sat in front of them on the ground.

Mo'ed Katan (16b): Irah ha'Ya'iri used to teach Yisrael while he sat on pillows and blankets. After he died, David sat on the ground and taught. They offered him pillows and blankets to sit on, but he refused.

Rishonim

Rambam (Hilchos Talmud Torah 4:2): The Rebbi sits at the head, and the Talmidim are in front of him in a circle, like a crown, so all of them will see the Rebbi and hear his words. The Rebbi may not sit on a chair and the Talmidim on the ground. Rather, either all are on chairs, or all are on the ground. At first, the Rebbi would sit and the Talmidim would stand. Before Churban Bayis Sheni, all became accustomed to teach while sitting.

Ran (Megilah 11b DH u'Makshu, brought in Kesef Mishneh): We say that Rebbi and R. Elazar b'Rebbi Shimon used to sit on the ground in front of Rabanan, who were on benches. Shimon ha'Temani used to sit on the ground in front of Chachamim. Also in the days of David, Irah ha'Ya'iri used to teach Yisrael while he sat on pillows and blankets! The Gemara answered that Moshe learned easy laws while standing, and difficult laws while sitting. We can similarly say that they used to sit on benches when teaching hard laws. If they would sit on the ground with the Talmidim, they would not be able to investigate them well enough. Alternatively, one may seat Talmidim on the ground only before they received Semichah. After they received Semichah, the Rav must show honor to them.

Lechem Mishneh: Why does the Rambam say that initially, the Rebbi sat and the Talmidim stood? The verse equates them. Also the Rebbi stands! The Beraisa said Stam that they used to learn while standing! I say that the Rambam learns from "Moshe learned while standing, and was Shoneh while sitting." Rashi explains that he reviewed his learning while sitting. The Rambam explains that he taught others while sitting. Therefore, the Beraisa means that they used to stand while learning, i.e. the Talmidim, but the Rebbi sat. How does the Rambam explain R. Avahu's teaching? Perhaps he explains that it refers to Talmidim with Semichah, like the Ran said. However, it is difficult to explain the Rambam this way. I.e. 'the Rebbi may not sit on a chair and the Talmidim on the ground' refers to Talmidim with Semichah, and 'at first, the Rebbi would sit and the Talmidim would stand' refers to Talmidim without Semichah. Surely his latter words refer to the same Talmidim as his former words! Rather, the Rambam explains that "stand here with Me" equates the Rebbi and the Talmidim. The Rebbi may not sit on a chair and the Talmidim on the ground. Even so, the Rebbi may sit while the Talmidim stand. This is not due to the Rebbi's superiority, rather, to enable him to help them understand. His mind is settled to explain properly only if he sits. If he sits on a chair and the Talmidim sit on the ground, this is due to the Rebbi's superiority over them. The verse forbids this.

Parshas Derachim (Drush 15): Rashi explains that David lowered himself and did not accept pillows and blankets. Hash-m rewarded him for this Midas Chasidus. Since David was able to investigate properly without them, letter of the law he could not accept them! Why does the Ran say that certain Chachamim taught on pillows because they needed them for hard matters? If so, also the Talmidim should be on pillows, for the Torah insists on equality! Also, why weren't Chachamim concerned that the Talmidim have pillows in order to understand the hard matters? Why did the Ran say 'if they would sit on the ground with the Talmidim, they could not investigate them well enough'? Why are the Talmidim relevant to this? Rather, the Ran explains differently. A Talmid is like a slave to his Rebbi, except for when learning. Then, the Torah decreed that he is equal. Sometimes this bothered the Rav, and he could not investigate properly. Therefore, he was permitted to sit while the Talmidim stand.

Parshas Derachim: Some ask why the Rambam said that the Rav used to sit while the Talmidim stood. The Prishah says that this refers to Talmidim without Semichah, like the Ran distinguished. The Lechem Mishneh is correct. However, how can he say that "Shoneh" means teaching Talmidim? The Gemara came to explain "va'Eshev ba'Har", when Moshe learned from Hash-m! Rather, it is like Rashi explained, that Moshe sat while reviewing the Halachah. Rather, the Rambam learned from Eruvin 54b. After Moshe taught Aharon, Aharon sat. Granted, if Moshe sat while teaching, this is reasonable. However, if Moshe was standing, how could Aharon, and later his sons, sit?! I bring another proof from Megilah 21a. It learns that one must stand while reading the Torah b'Tzibur from "stand here with Me." Even if Moshe learning from Hash-m was like Keri'as ha'Torah b'Tzibur, how can we learn from Moshe? He needed to stand, for he was the Talmid! We can answer if we learn from Hash-m, and we say that initially, the Rebbi did not stand. Hash-m stood only because this is a law of Keri'as ha'Torah.

Rashi (13b DH d'Chazisei): When Rebbi learned from R. Meir, he saw him from the back.

Rashi (Sanhedrin 17b DH Rav Nachman): Also Shimon ha'Temani did not get Semichah. He and the other four Talmidim listed [without Semichah] sat on the ground and gave answers to Chachamim.

Poskim

Shulchan Aruch (YD 246:9): The Rebbi sits at the head, and the Talmidim are in front of him in a circle, like a crown, so all of them will see the Rebbi.

Shach (9, citing Ein Yakov Megilah 21a): When they were Shonim (learning Mishnah), all stood, for they did not need to be so exacting. When learning matters that depend on reasoning, the Rebbi sat, and the Talmidim stood. Sometimes all of them sat, i.e. when they needed to be exacting and toil in Halachah.

Shulchan Aruch (ibid.): The Rebbi may not sit on a chair while the Talmidim on the ground. Rather, either all are on chairs, or all are on the ground.

Beis Yosef (DH u'Mah she'Chasav v'Lo): The Tur learns from David. Since he did not accept to sit on pillows and blankets, no Rebbi should aggrandize himself to do so while the Talmidim sit on the ground.

Gra (29): We learn from Kerisus 6a.

Note: Perhaps the Gra does not cite Eruvin 13b, because it does not explicitly say that Rebbi discusses seeing R. Meir while learning from him. Even though Rashi explains that it does, and the verse brought connotes like this, the Rif (Beitzah 20a) brings a Yerushalmi that connotes that he saw R. Meir from the back on one occasion, and it was not while learning.

Rema: Some say that this is only if the Talmidim have Semichah.