Your point would probably be better taken if you didn't pejoratively refer to COVID-19 as "a borderline cold", which it most definitively is not.
The real danger with opening up schools (at least at the HS level) is less about the kids who attend (99.9% of whom will be fine) and more who those kids may transmit the virus to (including teachers and staff who work there, and their families at home). You just can't 100% open the schools up to full capacity without an increase in transmission. Otherwise, I'm not sure how you convince teachers and staff to show up and teach in tight classrooms with 30+ kids, in HS's with over 3K kids roaming the halls.
But your points about the negative effects of having teenagers out of school is well taken. And I do agree schools should be open, in some capacity - whether that means split shifts, opening up additional classroom space, staggering the school day, implementing a hybrid (distance learning + live instruction) model, or whatever other creative solutions people in education are looking into.
And once those schools are open, you might as well have these kids playing sports. It's an important outlet for a lot of kids. ****, for some kids, the only reason they even go to school and make decent grades is for fear of getting kicked off whatever team they love. Get the infrared thermometers, check temperatures before every practice, pull and quarantine anyone displaying symptoms, implement social distancing during individual drills and film sessions, minimize fans in the stands (if any), and have their parents sign a waiver (or pull the kid from the sport if they want), and go with god. And anyone who thinks the risk is too great and wants to opt out, you obviously let them.