I can say it's against the law, because by doing so, you have violated RSMo 571.107.1.(10) (going back to schools.) Once you've violated the law, there is the issue of penalties. Those are set down in RSMo 571.107.2.
That section says that if you leave without a peace officer being summoned, you get away scot-free. And, it explicitly calls this not a criminal act, that's all unusual in the code, but my guess is that the state expects people to make mistakes, and if they admit their mistake, and fix the issue, there's no penalty. But, if you refuse to leave, then you can get in further trouble. You have still broken the law, it's just a very minor break, and nobody's going to make a stink if you then comply.
In any case, we're splitting hairs. I would not recommend going into one of the prohibited places in 571.107, regardless of the penalty. That's mostly because I want to remain on good terms in my community. And, I'm concerned that if breaking those statutes become commonplace, the "get out of jail card" will be rescinded. Evidently, you would not necessarily say that, because there is no real penalty. Maybe you place your 2A rights higher on your priority list than I do. Again, there's nothing wrong with that, it's just not where I place my values.