So they don't catch a potentially serious illness, have their education disrupted, and pass it on to multiple others with the effect of disrupting the whole school, so their relatives don't catch it and get more seriously ill and die, so they can meet their grandparents without exposing them etc etc etc.
The risk from myocarditis is higher from the disease than the vaccine. Vaccines were approved as having positive risk/benefit for 12yo+ in June.
I'd rather they were protected than abandoned, personally. The risk benefit for the kids is positive.