Let's list out the people we can rule out:
It's obviously not Toramaru because she was on the bench at the time
It's not Youdou, because this does not seem like the kinda shit she would do. She's a schemer, but I don't think she would ever assault him this violently. Also, she's on her shift so she's manning the station, so to speak
It's very unlikely to be either Yukata or Matsuri because not only would it be wildly out of character, but they'd be on other ends of the school and/or doing their own thing. The only way I can see it being either of them is if it's a confession gone wrong that made her go slightly insane, which would be utterly bullshit and super character-breaking for Yukata, or if Matsuri thought it would be a good opportunity to get rid of him/she wanted to use him for the confession thing. I think both of those are unlikely, given she's grown beyond the dependence on Toramaru and the fear that he'll replace her. I think she may be dumb enough to try and use him for the confession, but I don't think she'd want to betray Toarmaru like that and I think it would be dumb for the author to do.
The other band of delinquents because they said that they wanted vengeance on Toramaru, and kidnapping her boyfriend and possibly torturing him would sound like that, but it might be too far even for them.
It's likely its someone unknown. If you look at the height comparison, they absolutely tower over him and have long, wavy hair.