Bhagavad Gita

ಪಶ್ಯೈತಾಂ ಪಾಂಡುಪುತ್ರಾಣಾಮಾಚಾರ್ಯ ಮಹತೀಂ ಚಮೂಮ್।
ವ್ಯೂಢಾಂ ದ್ರುಪದಪುತ್ರೇಣ ತವ ಶಿಷ್ಯೇಣ ಧೀಮತಾ ॥೩॥
paśyaitāṃ pāṃḍuputrāṇāmācārya mahatīṃ camūm|
vyūḍhāṃ drupadaputreṇa tava śiṣyeṇa dhīmatā ||3||

Sloka 1:03
Gist of the sloka:
O Pandavas Acharya, take a look at the large formation of Pandavas army, who are being commanded by your disciple, the son of Dhrupada
The Commander of the Kauravas was Bheeshmacharya. Rather than approaching the commander with his concerns Duryodhana approaches Dronacharya and speaks to him within the hearing of Bheeshmacharya and that too in a arrogant manner.
He first addresses Dronacharya [Who taught both the cousins Kauravas and Pandavas] as acharya of Pandavas. Indicating that he suspects his own teacher of probably siding with Pandavas and indicating he is watching his actions.
Secondly, he implies to his teacher, not to show any pride that the Pandavas have learnt their lessons in warfare and thereby showing their skill in the battlefield formations.
Third, he highlights that the commander of Pandavas as “son of King Dhrupada” (Dhristyadhyumna); who again was the disciple of Dronacharya.
Duryodhana is already in deep doubts and has anxiety about his actions and winnability, is even more troubled with increasing doubts about everyone around him.
Dronacharya and Drupada were both disciples under Rishi Bharadwaja. As such developed friendship with each other and remained so until they graduated. While Drupada went on to become the King, Drona after various trails remained a poor person but a Gynani.
Dronacharya son Ashwatthama, once asked him for milk as he had not yet tasted it. Dronacharya wife used to mix rice flour with water and used to offer it as milk to the young son. It so happened Ashwatthama once tasted real milk at his friend’s place and realized he was not been given real milk at his home. He demanded from his parents for the same.
Drona remembered his friend is now the King and thought he could ask for a gift of cow from him. He approached the King in open court and addressed the King as his old child hood friend. The King indicated to him that he was no longer his friend as they were now at two different levels in the society. They need to be of equal status if they need to be considered as friends.
Dronacharya then, insulted in the open court, went towards Kauravas kingdom and became their teacher. Once the teaching was complete, the disciples asked Dronacharya what would he take from them as offering from his disciples – guru dakshina [fee].
Dronacharya asked them to win King Dhrupada and bring him before him – as a prisoner. Pandavas were successful in doing so. Dronacharya then returned half of the kingdom to King and retained a half. Thereby making them equal. He then sought the friendship of the King as they were now equal. The humiliated King, performed various sacrifices and obtained a daughter [Draupadi] and son [Dhristyadhyumna]. Dhristyadhyumna had taken to vow to eliminate Dronacharya as Dronacharya had humiliated his father King Dhrupada. Draupadi in turn married the Pandavas; the very Pandavas who had initially subdued King Dhrupada on behalf of Dronacharya.
The King knowing Dronacharya was the best teacher, had his son Dhristyadhyumna trained under the very person Dronacharya.
These factors did not stop Dronacharya from teaching Dhristyadhyumna the best he could. The exemplary ways the people behaved separating their personal issues with the duties of Teacher/Disciple is alien now.