i know from personal experience that this is an incorrect assumption
also you can probably expect to pay a fine and spend several months in jail for doing this, as well as a completely ruined personal record.
if you are not intelligent enough to think up a way to circumvent this problem more creatively than BREAKING AND ENTERING then your movie probably sucks anyway and you shouldn't be putting so much on the table for it.
man
do you really think they're going to send some like 16 year old kid to prison and ruin his personal record because he broke into a school to shoot a movie? he will get in a ton of trouble and i don't think it's worth it either, but in my experience that is not what they do for breaking into the school if you don't vandalize property, since most administrators will make the distinction. but schools vary quite a lot in this regard, and also in terms of how easy it is to break in, depending on the district and how much money the school system has to throw around. sneaking into some schools at night is really quite easy! some others it is essentially impossible without getting caught. i am not trying to sound like a BREAKING INTO SCHOOL AUTHORITY at all but yeah i actually had quite a bit of experience with this when i was younger, in multiple schools.
if you go to a really poor school district, or live by one, i would try to get in maybe, but really you should probably just ask OR wait like two weeks for school to start and do it then.