Yeah, it's almost certainly the excellent 'Worms', from Prog 598 (reprinted in Casefiles 12 - on the cover as it happens), with gory art by Chris Weston. A Dennis The Menace lookalike feeds classmates and teacher to hideously grinning giant recycling worms while on a school trip, then tries to convince Dredd that he was an innocent bystander. Great stuff.