Okay, this might not be as big a dilemma as I make it sound, lol. But whatever

In about a week I'm going to an LGBT event directly after school. That means I have to bind for probably 10-12 hours that day. In school I'm not out as trans, I'm just a guy there. Since I'm pre T and fairly big chested, being as flat as possible is extremely important to me.
Here's the "dilemma": what should I do about binding?
I have a very new binder (about one and a half month old) which binds very well, but it's extremely tight and I can't wear it for a full day without my back getting sore. Then I have some older binders that still make me somewhat flat, but not to the point where I'm confident enough to go out and meet people. But since they're older and more loose, I can pretty much wear them for a whole day without getting hurt.
I would prefer wearing the new binder the whole day, but my sense tells me not to, haha. I'm thinking about wearing one of the older binders in school and just layer a lot of shirts and put a sweatshirt over to make my chest less visible. And then change to the new binder and take off some of the shirts when school is over.
I don't really know. What would you do if you were in my situation?

Maybe I'm just over thinking, haha. I don't even know is my class mates would notice my chest, but I feel pretty insecure and dysphoric when I'm not flat.