(We can relate to the story of Drona. Forgotten school friendships are nothing new!)
Drona and Drupada, the prince of Panchalas, were good friends as kids. In fact carried away with this friendship Drupada promises, 'When I become king I will take you with me and we can be friends for life'.
Drona married to Kripi has a son Aswatthama. Drona's ambition was to become the greatest archer of the time. He went to the great Bhargava who had toured the world twenty-one times, destroying the Kshatriyas. Bhargava accepts Drona as a student and Drona comes home after acquiring mastery over all the astras.
Drona is in extreme poverty and they cannot even afford milk for his son Ashwattama. (The story of the Indian origin coaches?) He then remembers Drupada and his promise and hastens with his family to Panchala. Drupada drunk with power laughs at Drona and insults him for imagining that a poor Brahimin like him could be friend of a king. Drona with revenge in his heart turns to Hastinapura, where his brother-in-law is the teacher.
Bheeshma appoints Drona as the teacher of the princes. Several years passed in the education of the young princes. Arjuna with his diligence becomes the favourite student of Drona. Drona also is quick to promise, 'I have never seen an archer like you. I promise to make you the greatest archer in the world'.
Once Arjuna shows his prowess by killing the crocodile which had caught hold of Drona's leg.
Drona extremely happy teaches him an astra 'Brhmasirsha' both how to dispatch it and withdraw it. He cautions Arjuna 'This astra is too strong to be used on ordinary mortals. If it is aimed at poor ineffectual persons it will destroy the entire world. If there is a person who is either a rakshasa or a perverted deva who causes great havoc among men then, only then, should this be used'.
( What is a perverted deva I wonder! The story of the World in the brink of destruction keeps repeating! But I suppose it is what people like to hear or read. Imagine how rich Vyaasa would be in modern days. It is said that the creator of 'Harry Potter' is a Billionaire. Probably not, we are constantly reminded that Mahabharata should not be read at home!)