Real pranks are funny and tie back into something that your class did, something memorable that doesn't cost the school tons of money to fix. What you did, and what most "senior pranks" are, is vandalism.
It doesn't cost the school a ton of money, it costs them maybe 30 minutes with a shovel. We didn't do it deep with cleanup in mind.
What about replacing the grass?
They can buy seed or just fill it in with the grass that we dug out. I guess they're going to buy seed though since they cleaned up all of the grass we left.
I don't know what kinda crazy highschool you go to, but around here the schools would never be able to put in seed because people walk through the grass and it wouldn't grow.
So paying for sod and then the labor of putting it in and whatnot... now, that may not be alot and might be fine if it were a humorous prank and the administration enjoyed it, but your prank was merely vandalism.