What even did she do?
If her bullying him is severe like say emotionally breaking him by making him a laughing stock of the class.
Sure I can understand revenge.
But if it turns out it's just like kids going "alala you suck hahah" or whatever kids do, I don't know if revenge is valid, kids are just stupid anyways.
But so far all we've seen is this girl is now the one being bullied and beat up. Badly.
Either she deserved that or someone else took control of the class and kicked her out the group.
Or maybe she was like a follower and they deemed her unnecessary and is now using her as a plaything.
Ultimately so far she's more of a victim than MC.
But we don't know what happened to MC.
So for now I'm just gonna assume the revenge is to make her marry him and have her suffer the life of a mom with loud and energetic children.