Manava Dharmashastra 2.197

2.197

tathā parāṅmukhasya guroḥ sammukhopaviṣṭaḥ śiṣyaḥ | yadi guruḥ parāvṛtya kathañcitsthitaḥ preṣyati, tāṃ diśaṃ gatvā'bhimukhībhūya pūrvoktaṃ kartavyam | dūrasthasya samīpaṃ antikaṃ etyāgatya prāpya | āsīnasyāpi śayānasya praṇāmya prahvī bhūtvā gātrāṇyavanamayya | nideśe nikaṭe tiṣṭhato'pi praṇāmyaiva yatprāguktam ‘abhigacchanniti’ |

Facing him, when the teacher has his pace averted; going near him, when the teacher is at a distance; bowing low, when the teacher is lying down, as also if he is standing close by.—(197)

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).