I would talk to the school again. Get the principal to send out a reminder that marked handicapped spots are for people with plates or tags ONLY. If you need a tag, please speak with your doctor, we have a number of parents with tags that are unable to get their students to class because of people illegally parking in state disabled spots. Police will be doing random patrols and ticketing those who are illegally parked in state disabled spots.
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Most people think I have a pass when I'm having a bad pain day (about half the time, the other half no one has a clue) and I'm doing the Will Smith Old Person Walk (so, so sexy!). I DO actually qualify for permanent plates (for the past 10 years, and q'd for temp tags on and off for over 20), but have just never bothered (since it's intermittant, and I'm lazy. I'm *supposed* to be keeping the muscles as strong as possible, and even on good pain days I'd cheat to feel less pain), although I came close this fall when I was *seriously* gimping it (reconstructive surgery on 1 shoulder, and a full brace on one of my legs... I was House for Halloween!... after my sling and brace came off and I was just using my emergency cane). My best friend HAS tags (and gets sooooo many dirty looks). She has a degenerative spine, and it's fused halfway up her back... but she can still run when needed, and is SUPPOSED to (like me) on good pain days to keep the muscles strong as possible. So, like you, I'm cautious about saying anything to anyone EVER....
BUT... while I'll occasionally stand in a spot (engine running, car in gear, able to move the HAIL out of the way for someone) for quick drop off or to turn around in a tight spot VERY occasionally... those blue squares are NOT my personal drop off zone, nor anyone else's.
TAGS or PLATES. Period.
((In my state -check with yours- 2x ticketed for parking in state disabed spots means LOSING YOUR LICENSE. That would be a lovely thing to have in that note from the principal if it's true in your state, as well))