You should be showing up at Public Comment during your town's next school committee meeting and telling them all this. You're not wrong, but my prediction is you'll get nowhere. I never said it's a bad idea. I said your town is not going to want to spend money on it.
My hallway has eleven classroom doors. There are six hallways in my building, plus about twenty additional doors on each of three floors. That's 126 classroom doors, and that's just in one building. And minus offices, utility rooms, etc; all should be hardened similarly to be effective. I'm not counting connecting doors, and many of those have zero deadbolts right now.
I have no idea how much it costs to buy an institutional lockset and have it installed, but I'd assume it's at least $100 per door (I can see it being double or triple that, but I don't really have a clue). For easy math, we'll say $100. That's $12,600 minimum for my building, and a couple hundred thousand overall for the whole district. Minimum.
That's not chump change, especially considering what you'd hear from the school committee:
1. The chances of a school shooting are tiny; that's too much to spend.
2. Each door already has a deadbolt. Why add another?
3. We're laying off staff already this year. There's no excess money.
By all means, search for grants. But be aware that many school systems (and almost all towns) pay someone to search for grants; they're probably better at it than you are.