Noelle knows what happened to Dess?
Supposedly, Noelle failed to spell "December" during that spelling bee because it reminds her of Dess. But she wouldn't be so traumatized that she can't spell a single word if, from her perspective Dess just "went missing", right? Worried sure, but trauma?
And surely, if the mayor's kid went missing, the spelling bee organizers would know not to put "December" as a word, right?
My idea is that Noelle saw what happened to Dess, but people only discovered she was missing after the spelling bee. Why isn't Noelle speaking up about it? Maybe it was so horrible she couldn't recount the events exactly? Maybe she was actually involved and is scared of confessing?
To apply a bit more "meta" reasoning, the Fun gang doesn't have any real attachment to Dess. When we finally hear about what happened to her from Carol or the Knight or whoever, it would just be an exposition dump on some girl that the casual fans who don't keep up with theories never thought too strongly about.
On the other hand, if we hear it from Noelle, and find out she was present or even involved in what happened, it would leave much more of an impact.