Thank you everyone for your replies!
I had sent an e-mail to School A, asking them (really politely) whether I could be given an extension, but they said "NO"
To make matters worse, School A is not a member of CGS. (while School B is.)
I really don't know what I should do...
I get the impression that it would be difficult to back out of School A later on after I've accepted their offer.
I've heard nothing from School B up to this point, and since School B is a member of CGS, I think it is quite unlikely that I will hear from School B soon.
I guess I should have my fingers crossed and wait
Good Luck to everyone with their applications !