Drona became the Acharya or Teacher of Pandavas and Kauravas after Dronacharya showed his expertise with bow and arrow. Legend has it that Bhishma, the grandsire of Hastinapura, was in search of an efficient Guru to teach Pandavas and Kauravas the art of weaponry. When the incident was took place, Pandavas and Kauravas were children and were living in Hastinapura.
One day Pandavas and Kauravas were playing with a ball. Suddenly the ball fell into a deep well. Children did not know how to retrieve the ball and they crowded near the well.
Dronacharya was passing by and the commotion created by children caught his attention.
When he found out the reason for children crowding near a well, he promised to help them.
He then took his bow and arrows and shot an arrow on the ball. He then shot another arrow on the end of the previous arrow. Thus a line of arrows was created by Dronacharya. Soon the ball was retrieved from the deep well.
The children who witnessed this amazing feat ran to inform about it to Bhishma.
Bhishma immediately realized the potential of Dronacharya and appointed him the Guru of Kauravas and Pandavas.