For Simpsons Skateboarding,at the character select screen,while holding L1+R1+L2+R2,bash this in:
Triangle,X,Circle,Square
Now do any challenge and go onto Shop.Put Jump to 99,then go onto Challenge 1 or 4 of Springfield Elementary.Now skate forward,then go back,ollie up the huge ramp on the left,and you should jump over the fence.Now don't go under the bar or you will be outside the fence again.Go to the back of the train-like thing and go inside.You can do some serious jumping.
OH,you can't go in on Skatefest cos' the door will be closed.