The level of sexual activity and number of partners may be lower than people in the early 20s (btw, that's certainly not universally true). But that's only one part of the transmission story. The other part of the story is asymptomatic transmission due to a lack of regular STI testing and access to adequate treatment.

In other words, high schools are particularly high risk environments not so much because high schoolers are more sexually active or have a greater number of parnters per se, because also and perhaps primarily because high school kids typically have a higher barrier to accessing regular STI testing and receiving STI treatment.

Why are you so confident? The 15-19 age bracket has nearly the highest prevalence of STDs: https://www.who.int/publications/i/item/9241562889