Ontario schools only:
You received an offer from School A but you want to wait for other Ontario Schools
provisionally accept school A and pay the deposit ( do it before the deadline. There may be a different deadline for offer and deposit)
You can wait for all other schools until late June or early July. I think it is July 2 or something. then your provisional acceptance will automatically become firm acceptance. Your applications to all other Ontario will be withdrew. Note : Only apply to Ontario schools. You can still wait for all non Ontario Schools.
If you really don't want to go to School A and want to take a and wait for other Ontario schools, then you decline in the provisionally accepted offer and forfeit your deposit. This is a risky move though.