Reading all the discussions.
I know that in the Mahabharata, there is some mention of Karna being Drona's student. However, that is one line in a thousand verses. Logically, it does not make sense to me.
If Karna was friends with Duryodhana from childhood itself, why would they form a friendship in the Rangbhoomi? Duryodhana didn't know his name too.
Also, if Drona accepted students from all of society and was not restricted to Kshatriyas, then why did he also take Ekalavya's thumb?
My personal opinion is that Dronacharya specifically had a motive to avenge his insult by Drupada and hence chose the princes of a powerful kingdom like Hastinapur so that the odds of him getting his revenge would be good. He was not interested or inclined to take any more students.
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