Difference between jumping on grass and in a school?

It's much the same. I think he would be fine to jump on grass for the first time at the show- you probably won't notice the difference. Only it can get slippy, so if it's raining (which it always seems to be whenever I go to a show!!) just go steady. Being barefoot will actually help to stop him slipping so don't worry about him needing shoes :)