go to the lake where the crown/kermit island is

go further toward the mountain and find the beach where the big sand gator is. its pretty big and easy to spot.

look underneath the sand gators mouth. there should be a crashed helicopter with a frog skeleton. steal the skeletons hat, but be quick when leaving, because there's a special surprise waiting for you on the way out!

equip the hat in the hideout or illuminati menu. now go past the swindon sign and follow the road to flight school. now you should be allowed to go in!