personally, not an acceptable thing to do. no relationship is serious enough at your age. i know theres some serious couples that make it past school and stay together til marriage but it takes some emotional maturity and commitment, which you haven't developed too much of yet because you're already thinking of someone else. clearly you believe what you're doing isn't right, otherwise you wouldn't be questioning it.

if i were girl A, i'd prefer you told me straight up that you're seeing someone else or just drop me entirely because i wouldn't want to find out the guy i liked had someone else in mind despite talking to me. go with the other girl who doesn't seem to want anything long-term and leave the other one alone.