I get what you're saying, but there's a nuance I think you're missing. Masks catch 90% of exhaled droplets. So you're still being exposed, but to 10% as much virus as you would be without the masks.

For the math to follow, I'm assuming that you have a six hour school day, a half hour lunch, are around the same number of people all day, at the same distances, and that everyone is maskless for precisely that half hour. So the real number would differ, but probably not by too much.

Without the mask mandate, you receive 6 hours of viral exposure.

With the mandate, you get half an hour of full exposure, and 5.5 hours of 10% exposure. 5.5 x 0.1, so that's 0.55 hours equivalent full exposure. Add lunch to that, and it comes to 17.5% of the exposure you'd get without a mandate.

How many viral bodies enter your lungs has bearing on the probability of infection, and the severity if you do develop an infection. High initial viral load is a major factor in how dangerous/deadly it is.

The mandate isn't perfect, but it's far from pointless.