It’s a thorny issue. I don’t believe it’s necessarily a threat, or a signal of racism, or any of the other black-and-white views about the flag that inevitably get spouted. It can be, of course, but so can damn near anything else in this old world.
And I agree with regulating what kids can and can’t wear into the school building, or in any other school-related situation where people are going to be closely together for extended times and everyone will be able to see your shirt, hat, banner, whatever. I don’t like it, but sometimes practical concerns have to trump ideals.
A bumper sticker in the parking lot, though, I don’t know that there’s a lot of practicality to banning it. I mean, it’s out there in the parking lot all day, while the students are, presumably, in the freaking school. Someone might see it for a minute or two coming in or out in the morning, but it’s not even remotely similar to the sort of exposure they’d have if the kid was wearing it on a shirt in the building with them all day.
As for the possibility of threats, intimidation, or violence…if this kid is going to threaten, intimidate, or attack black students, he’ll do that no matter where his vehicle is parked, no matter what decals are or aren’t on it. If black students are going to threaten, intimidate, or attack him because of his purported racism, they’re going to do that no matter where his vehicle is parked, no matter what decals are or aren’t on it.
So right now I’m not really seeing any practical advantage to banning this bumper sticker from the parking lot, certainly not anything strong enough to trump freedom of speech. I reserve the right to re-evaluate if further relevant specifics come up, like a history of racial violence in the school parking lot, or kids being able to mill about the lot during the school day, or…well, you get the picture.