Okay, I was worried there at the start. I didn’t know what was going on and I found that basic shoujo reverse harem so boring (not that I hate shoujo, but that just seemed like the most generic shoujo; generic shounen is just as boring).
Also, that conflict cannot be more artificial, forced, and just kinda stupid. 1) It’s sexist to assign roles in helping the class based on Sex. That’s why we’ll never get sexism out of our society because it’s ingrained in basic stuff like this. The boys shouldn’t all even have to do physical labor. There should be a volunteer system and if there aren’t enough volunteers then start picking people based off of athletic or physical ability (or randomly). From my understanding of the cultural festivals they’re meant to be fun and enjoyable anyway. I don’t see why wouldn’t just let people help where they wanted to help. 2) That teacher isn’t completely unreasonable (or doesn’t seem like it; maybe a bit sexist, but not really actively trying to be that either). The MC arguably has to do MORE work than any single guy in the class already just by having to design and make an entire costume capable of winning. Just talk to her and get out of those duties. If after the talk she still forces you to do both things, then don’t give her the satisfaction of a home made costume and all the time, work, and dedication that goes into it. She seemed competitive, but if it comes down to the MC having to overwork himself for it, then just buy a costume and let the teacher deal with losing (but again, she didn’t come off as actively unreasonable).