This story must have been told and retold hundreds of times. A Nishada (Tribal!) sees Drona teaching the Khsatriya princes, approaches and requests to be taught. He is sent away, kindly as per Kamala's version and rudely in Samhita's version. Undeterred, the boy Ekalavya makes a figurine of Drona out of mud and installs it as his guru and practices archery. He in time becomes a master in archery.
Inevitably, a dog that belongs to princes wanders into the forest and barks at Ekalavya. Irritated, Ekalavya seals off the mouth of the dog with arrows. The dog runs back and princes are astonished to see the skill with which the arrows have sealed the mouth of the dog, retrace the path of the dog along with Drona and meet Ekalavya. They ask him who his guru is and are shocked when he says it is Drona. The boys and also Drona are upset that a Nishada is a better archer than Arjuna. Drona then seeks Guru Dakshina from his pupil, his right thumb, which would effectively prevents him from being a better archer! Ekalavya readily obliges his guru by cutting of his right thumb.
(This story is best viewed as the one that probably reflects the period. This terrible act was the result of Drona's rash promise to Arjuna to make him the best archer in the world. What prevented him, I wonder, to put the challenge back to Arjuna to do better than Ekalavya. Drona seems to be a flawed person. Earlier, we see that he practically deserts his wife to improve his skills with Bharadwaja, leaving them in utter poverty. Now, he demands a ghastly fee from an innocent boy!
Probably he was afraid that he would loose his job, as coaches normally do now when their team fails!
This also reminds me of a story where the best person in a sport was ignored and and one well connected was selected to represent India in international meets!)