Can I drive a school bus in there it is empty and is there a fee to get in I wanted to get to the beach area

It'll be tight on the access road. I'd call the park ranger's office and ask. The parking should be fine but the access road will be very tough if there's any oncoming traffic. You will need a pass (you can buy from ranger's office) to access park.

Definitely remember riding into the park in a school bus for for field trips as a kid. So I would think there shouldn't be an issue with that, as far as fees go you will definitely need to get a park sticker if you don't already have a Wisconsin state parks sticker. Don't know if there is a different fee for a vehicle as large as a bus. You can get a day pass but I would definitely just recommend getting the annual park sticker that way you can enter any State Park!

There is a fee to get into the state park. There isn't a big beach area at Potawatami. Peninsula and Whitefish Dunes have more beach, as does Sunset Park in Sturgeon Bay. Some roads in Potawatami have some tight curves. You'd have to check with the Park Rangers when you pay for your fee what roads you can take a bus on.