The shell your classmate's crab is in looks like an apple land snail shell (not to be confused with Apple murex) usually grey or green, found in swamps and sold super cheap here in the U.S.. If he is a jumbo FYI finding Apple snail shells his size may be hard even though they can fit larger crabs easily, Jumbos are a bit more than just "on the larger side". My A. LSS (personalize abbreviation for my notes) are about 1.5 inches and can get bigger I'm not sure about 2"+ though.
Mommy of 11 PP babies - Poseidon (Sid), Toby, Matty, Junior, Aries, Kaylee, Turbo, Adrian, Freckles, Pepper and Flower. RIP Octavius & Fred