I am in a similar situation. I have let the School A know about the admit from School B (without naming it) and asked for an update on my application status. If I don't get an acceptance from (School A) anytime soon, I am leaning towards accepting the offer I have (from School B ). "A bird in the hand is worth two in the bush." Though, it might not be very uncommon, I am not able to bring myself to ask for an extension from School B, more so as it is offering me funding as well.
Wish it wasn't as difficult as it is. *sigh*