That's an interesting question. A quick search on Derpibooru showed that Rumble was in a couple scenes with the other schoolkids in Season 4, but I couldn't find anything more recent. Maybe they moved as a part of Thunderlane's Wonderbolt training? Or maybe Rumble switched to homeschooling? I don't know. It would be nice to see him pop-up in the show again though. Thunderlane got his turn, so now it should be Rumble's turn.