I've been thinking about who could possibly be blackmailing Chad, but it may be yet too soon to point to a culprit, as we lack much information to reach a solid conjecture. It could be some character not yet introduced, like a rival competitor of his, but it makes more sense for it being one we already know. The suspects could vary, depending on what the content of the envelope is. I'm assuming it's compromising pictures, kinda like the ones in Dawe's envelope, pertaining his secret love life.
Gathering all the clues we've seen so far, it makes sense that the content of the envelope shows pictures of him and his lover, which probably is another guy. This would fit well with his behavior, because he has not shown much interest in Sage (especially of the sexual nature), except for when they're in public. This could mean that Chad is living a double life; one for the public's eyes, and one he doesn't want anyone finding out, over the threat of his reputation being tarnished, and having consequences for his social status.
The Jocks appear to be fully on his side and haven't shown any evidence of jealousy or of holding grudges. The DIKs, however, have plenty of reasons/motives to want to hurt him, with them being rivals, and especially after the whole Tommy/Arieth ordeal. But the DIKs don't seem to have any personal issues with Chad himself, but more with Dawe and the frat as a whole. But Chad is/was the leader of the Jocks and Dawe was also targeted by the blackmailer(s). So, a DIK member could very well be the culprit; but this would be way too obvious, imho.
The Preps don't strike me as suspects either, because they could have more effective means to deal with the Jocks, and they don't have any (apparent) monetary reasons for blackmailing them either. The Nerds could have something, since they are bullied constantly, but we have no specific clues in that regard. And the HOTs are "associates" of the Jocks, but it is mostly here where my suspicions lie. I can see someone there with a motive, like, for instance, someone who didn't handle rejection very well.
Outside the frats, I have no idea who it could be, so far.
Edit: Oh, there's also Troy.