Manava Dharmashastra 2.198

2.198

nīcamanunnatam guruśayyādyapekṣayā ca nīcatvam | nityagrahaṇādbrahmacaryāduttarakālamapi | gurośca dṛṣṭigocare, yatra guruḥ paśyati, tatra na yatheṣṭamāsīta, pādaprasāraṇāṅganiṣṅgādinā | āsanagrahaṇaṃ ceṣṭāmātropalakṣaṇārtham | yatheṣṭaceṣṭo na bhavet |

Medhātithi’s commentary (manubhāṣya):
‘When the teacher has his face averted,’—the pupil should sit facing him; that is, if the Teacher happen to have his face turned the other way when he gives his directions, the pupil should go over to the side facing the teacher and then do the aforesaid (listening and conversing).