I'd prefer if you said "are you still going to school?" or "do you still go to school?" All questions are grammatically correct, but the first question could be misunderstood depending on when and how you ask someone. If you're a pen pal with someone and email them saying, "are you still in school?" the pen pal could happen to be a teacher and say yes. Another way to ask is by asking, "do you still attend school?" The use of the word 'attend' makes your question more clear! ^^ You could even say: "Are you student or did you graduate already?"

For me i always say "Are you still a student??"

grammatically, all three r right. No difference.

"What do you do? Are you a student?" this works for me all the time. :)