You seem like missing Alexis had a chance to talk with MC. If she had the chance, Erynn had it too. No need to blame any of them, they were children after all but saying Erynn is blameless in that situation would be willful ignorance at best. Sure MC might have picked a wrong time but leaving MC like that to look at whatever Ethan was showing was wrong. She could have said one moment to them instead of MC. As others pointed out Ethan was constantly bullying MC and she kept being friends with him was wrong. After the confession and Ethan making fun of him, MC stopped talking to her. She is not that stupid to not understand it was because of that interaction. She could have tried to fix things. She is constantly ambushing MC while he is sleeping, as they said she was doing that while they were children too. She could have done that but she did not. That was a mistake too. She said if you don't stop we are done to Ethan, he did not stop and she did not keep that promise and kick him out her life. That was a mistake too. MC told what happened to Emily, Erynn could have asked about it to Emily and clear things with MC but that did not happened either. So, no she handled entire thing terribly.
Was MC blameless? Of course not. As far as we know he did everything to avoid Erynn. Also, we can deduct from his behavoiur if Erynn did everything right but refused to date him, he was going to do everything he did anyway. Also, let's not forget he did all of it as a hail Mary. He noticed Erynn was getting away from him with her new friends, so confessed to him. That's hell of a mistake too.
So, all parties handled things terribly. Also, it's not easy who did more wrong and even if we do it would not be more than personal opinion either. In anyway, kids do these things, sometimes adults too, if they can make amends it's good if not tough luck, no need to drag this on.
P.S. I still do think there is more about this whole situation though. Something Erynn is not sharing. Also, I think Erynn was going to refuse MC even if she had some feelings. At that age, a few years is huge difference and usually kids in high school see kids in elementary as toddlers. You can see Erynn feels like at too at the beginning with her hair ruffling MC. Btw, I think if Erynn was dating Ethan at the time, all of it would make more sense. Though, I think dev could not do it because of possible backlash of NTR people.