re Trump vs BLM flags, the former refers to a specific person of a specific party and that flag is
literally a representation of fealty to same.
In the latter, the words that comprise the phrase cannot be contested (but actually are, which only increases the validity and urgency of the phrase). If you want to throw in another color or flavor there then fine, because that statement comes from a position of inclusivity. Yes, all lives matter but we all know that life ain't fair and certain groups of people have it less fair than others.
For many, unfortunately, the phrase BLM matters too because they've been conditioned to associate that phrase with purely negative connotations. So for those individuals, seeing a BLM flag they see the flag of their enemy and that, of course, must not stand.
So to compare and contrast vis-à-vis Trump vs BLM, with the former you have "Hail to The King" and the latter being "let's end police abuse of power" that has been rebranded as evil by its opponents. They're coming from completely different intentions and intention matters a lot.
BTW, I wouldn't support Biden flags in the school either.
But putting up a flag that might comfort LGBQT youth in a world that is hostile to them? Or a flag that explicitly states that they matter? Your homework assignment is to compare and contrast those flags.