Just to let everyone know, my daughter's entire middle school is going up to IOA on Friday May 21st (about 2k students). The good news for you, is that the busses usually arrive around 9:30am, and by the time they get off the bus, organized, and enter the park, it's 10am.
Not that it effects the Mummy event, but if you plan on going to IOA on Friday first, I'd suggest you get there early. The kids won't have any way to get express passes, but if a big group decides to tuff it out and wait for a ride, you may be waiting as well. Last time my daughter went, they didn't even get on any rides because the lines we're so long. They just walked around, played games, and bought candy.