Rules are fairly loose in public (meaning government-run) schools in the U.S. with regard to teachers bringing their personal political and social views into the classroom. It varies state to state like it always does, but most places allow teachers to display religious and political symbols within certain parameters (can't be distracting, can't be included in your coursework, can't be hostile to your students' own opposing views.) Texas I think allows teachers to have religious items on their desks only, and on their person as long as it isn't attached to their I.D. badge.

It says nothing good that my immediate thought in this case was to try to remember what the rules are for religious displays 😞

I agree with Jules that this is obviously intended to kick the hornet's nest in this town. Pride flags are associated with a hostile invading culture where I'm from too. Lots of people from more liberal areas move here because our economy is healthier, and people feel a tad colonized. This guy wants to be a martyr.