The Talmud (Megillah 21a) relates that from the days of Moshe through those of Rabban Gamliel the Torah was only studied while standing. After Rabban Gamliel's death, "sickness" descended to the world and people began to need to study while seated.
It further relates (based on the verse Devarim 5:27) that a teacher should be on equal standing with his students when teaching. The Shulchan Aruch (YD 246:9) codifies this, ruling that a teacher of Torah should not sit on a chair while his students sit on the floor. Rather, either they all should sit on chairs or all sit on the floor. I note that different distinctions are brought in the application of this particular rule (regarding the difficulty of the material being studied or the caliber of the students), but no one there speaks out against sitting on the floor.