Lol that's been around forever. My friends and I did it when we were teens. It will give them blisters on their tongues and cheeks if they do it too much. I didn't get blisters because I'm a wimp, but a couple of my friends did, they used salt-water instead of mouthwash for a few days to avoid burning and infections from food and mouthwash.

Not that I encourage being stupid, but teens are dumb and there are worse things they could do.