The people who may have coerced them into doing it. If a 30-year-old does it it's grooming, but if an 18-year-old does it to a 13-year-old it's not? I don't buy it. Seniors come from a place of power in schools like that, especially sports team seniors in sports-focused schools.
Why does it have to be grooming and coercing?
This sounds like it was a really well-known popular thing that students just willingly wanted to get in on. I'm not saying a 12-13 year old student wasn't forced or tricked into doing it, with peer pressure or whatever, but that's not the only explanation.
Also, I don't see anything that says anyone is 18 or older.