To be honest, it depends on how responsible a student is. I myself is a still a student who relies in Bitcoin to help me finance myself but I for once haven't forgot about my studies and had an average grade of B+. You should consult your students and approach them to give them some advice. It's not Bitcoin's fault but it was students entirely. 
I agree with your opinion mate. I even convince my friends and neighbors that are highschool students but they are not affected with their studies because instead of interacting with Bitcoin they choose to hang out with their classmates and friends play basketball or going somewhere just for fun. At first they're interested but as time passed by they abandon what I have taught to them. The same as what had happened with OP as he is now concerned about the performance of his students most especially if those students are earning much money from Bitcoin but still depends on how they managed their time of course.