Nah I kind of get it

I think Dan is staying quiet for 3 reasons:

First its her personality she really dislikes conflict and drama so she doesn't ever want to make it worse by engaging with the snakes and more over she doesn't want to make things worse because if she denied the stuff they said people would probably say well of course you would say that bet she's lying or look how egotistical this girl is. The point is that no matter what she says or does people will have their own opinions and twist it so she looks like the bad guy

Secondly she doesn't want to cause any huge ripples cause she's still a side character in the story so she's worried about how it could affect the main narrative, remember right now she just wants to graduate high school then the story will be over and she can be "free" to be best friends with the others

Thirdly it's a case where this girl cannot catch a break, and she's just overwhelmed by all the bullying, emotional manipulation, girl needs room to breathe. When you're in that situation and you're a naturally introverted person it's really easy to regress into yourself rather than argue especially if she's continually worrying.
She's constantly toeing the line of remembering this is all fictional but this is also her reality. In that way I feel like she may feel disconnected to.the bullying cause she's aware it's happening but she's also very much so an Outsider. When you look at it from that perspective tbh I'm surprised the girl hasn't had a mental breakdown yet. I know probably would have ╥﹏╥