Bhashya
उपकोसलो ह वै नामतः कमलस्यापत्यं कामलायनः सत्यकामे जाबाले ब्रह्मचर्यमुवास ।तस्य, ह ऐतिह्यार्थः, तस्य आचार्यस्य द्वादश वर्षाणि अग्नीन्परिचचार अग्नीनां परिचरणं कृतवान् ।स ह स्म आचार्यः अन्यान्ब्रह्मचारिणः स्वाध्यायं ग्राहयित्वा समावर्तयन् तमेवोपकोसलमेकं न समावर्तयति स्म ह ॥
In plain words
Upakosala, son of Kamala and hence known as Kāmalāyana, lived as a celibate student with Satyakāma, the son of Jabālā. The word 'tasya' here indicates a traditional account: for twelve years he diligently served that teacher's sacred fires. Yet while the teacher would have the other students complete their Vedic studies and send them home to their families, he consistently left Upakosala behind without sending him home.
Close to the Sanskrit
Upakosala, by name the offspring of Kamala, hence Kāmalāyana, lived as a student of sacred knowledge under Satyakāma, the son of Jabālā. 'Tasyāha' has the sense of tradition: he tended the fires of that teacher for twelve years, performing the service of the fires. That teacher, having made the other students of sacred knowledge grasp their Vedic recitation and sending them home, did not send Upakosala alone home.