So Prompto has to be a scholarship student or something to be able to attend the same school as Noctis. I'm assuming that the children of other elite families and whatnot also attend this school. So lets say it just ticks or rubs them the wrong way that, Prompto (a nobody, someone who isn't from a prestigious family or anything)comes along and becomes BFF's with the prince.

They begin to bully and pick on him (when Noctis isn't around). Prompto keeps quiet about it, he doesn't want to cause trouble for Noctis, especially now that he actually became friends with him. I imagine that maybe there's one particular bully (bully could like Prompto and is pissed he's always with Noctis, idk) who has it out for Prompto and does worse things each time (just touching to eventually attempted or full on noncon). The bullying gets worse and shit hits the fan one day when Noctis is absent (princely duties or something idk). Noctis and the bros find out and are furious.