> There's no evidence that wearing masks prevents contracting Covid (or any other airborne virus). It does a bit toward preventing spreading it if you already have it
What you just described is the same thing, it's simply a matter of perspective. If you wear a mask, it helps me by reducing the chances that you spread it to me if you have it but aren't experiencing symptoms yet. It works the same way in reverse. I don't wear a mask to protect myself, I wear it to protect others in the case that I'm shedding virus and don't know it yet.
> It does a bit toward preventing spreading it if you already have it, but if you already have it, the schools send you home anyway
Most of the schools that do testing due so weekly. With delta someone can be contagious up to 36 hours after infection. That means someone who gets tested on Friday morning, goes to a poorly ventilated party with a COVID-19+ that evening and returns to school on Monday could spread the disease for a week.
There's no evidence that wearing masks prevents contracting Covid (or any other airborne virus). It does a bit toward preventing spreading it if you already have it, but if you already have it, the schools send you home anyway. It's also not as if masks are themselves neutral either - constant mask wearing is associated with higher rates of bacterial pneumonia. Even medical professionals never wore masks all day (until last year), just when they were performing surgery.