This really depends on the specific school. In our area, some schools have low enrollment, so even if kids get disenrolled for missing too many days, they can just reenroll and return to the same class. However, our school is very full with a long waiting list, and kids that miss too many days get actively disenrolled and may not be able to return to the same school. Our kids are little (oldest is in 2nd), but they love their school and we will maybe miss a day or two at their current ages. As they get older we’d rather avoid missing days if possible.
The good thing is, I’m pretty happy with their school calendar. In addition to summer break, they get full weeks off for Thanksgiving, February break, and spring break, plus 2 weeks off for Christmas/New Year’s. We’ve been doing around 3+ weeks/year each at WDW and DLR, sometimes swapping out for a different destination. The trips are pretty much all during school breaks (we took them out for 1 day in the fall so we could go to Oogie Boogie Bash - so worth it!).
It can be quite crowded during breaks, but we’re also used to it now, familiar enough with everything to navigate crowds, and also know that we’ll have another trip to do things we may have missed. So we still enjoy our trips during breaks despite crowds and with
Genie+, we generally avoid long lines. Some of our favorite days, though, are days where we hardly do any rides and just soak in the atmosphere doing non-Genie+ focused things.