shouldnt be a problem. usually the deadline is negotiable. i am suprised your school didnt give you an extention.
The best way to cover yourself is to send them an email and ask if it is possible to give you some extention since you are waiting for another school's decision. If they say no and you turned back on them for the same reason, it is not your fault.
no worries. you are only risking your deposit money though. | 
