Go to the cafeteria and talk to Jake all the way to the right, he's the dude who's about to hurl his lunch. He talks about wanting more anchovy chips.
Go through the cafeteria kitchen door, there's a quarter on the window sill next to the door.
Take it to the dude in front of the vending machine who also wants anchovy chips, two bags will drop, he'll give you one and just take and give it to Jake.
He'll hurl up the prom proposal, grab it, give it to Jake to sign, and just take it back to Crystal in the women's bathroom.