The idiotic brag is real.

Yeah, I mean, who WOULDN'T brag about getting held back?

"Humble brag" is an oxymoron, I think subtle is the word you're looking for.

I think the point of the term "humble brag" is to call out subtle bragging in what initially seems to be a show of humility

A call would work, and don't call me Shirley.

I don't see why he wouldn't have though, only one mark below 90% is not bad at all.

Probably is a scare tactic.

In a way they are, but really, failing a student because of "lack of parent teacher communication?" My parents almost never speak to my teachers (and if the teacher needed to talk to OP'S parents so horribly that they are going to fail OP, why didn't they tell them to talk to them during a parent teacher meeting?) and I have never been threatened to be failed because of it. It doesn't make sense to fail a student because of that.

I don't think it's the lack of communication as much as they're at a stalemate. The parents (who are the idiots of this situation) somehow refuse to sign op's report card because they are unhappy about his grade (as if a refusal of signature is going to make the teacher change the grade) and the teacher has to have them sign the report card in order for it to all be valid (I guess). Sounds like the parents are being idiots. A refusal to sign the report card isn't going to change the grade. I'd have the teacher try and explain that to them (and that a grade just under 90% is an incredible grade) and see if they'll change their minds. Good luck op and sorry you're in the middle of this. If all else fails, try forging it yourself and hope the parents forget about it.

I don't think being a minor has anything to do with this situation.

Being a minor means that the parents have the control. If the OP was not a minor, this would not be an issue.

Definitely makes a difference! Can you imagine a professor at university calling someone's parents about an exam grade?!