I am actually on a waitlist too. Here is what the professor there told me.
"If you have an offer on the table from another university, you can ask for a couple of days extension."
If that school isn't willing to give you the extension, I guess you will have to make your decision by Friday then. Nevertheless, accepting one offer doesn't automatically mean that the other school would know it. However, you will have to sign some paperwork (or electronically), so it might be hard to get out, if you later find out that you also get the offer from the other school.
Good luck!!