Guru Drona's guru Parashurama was against teaching to Kshatriyas. However he still taught to Bhishma. In the similar way, Drupad and Drona were considered Guru-Bhai; probably that means Parashurama also taught to Drupad. Why?
Guru Drona was the son of rishi Bharadwaja. Both Drona and Drupada studied under the guidance of rishi Bharadwaja. Also, when Guru Parshurama was giving gifts to all brahmans, Drona reached there. Only the weapons of rishi Parshurama were left. Hence, the weapons and the knowledge to use them was given.