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.

I think the important thing to remember is that safety comes first. Personally, I'd rather take the binder that's kinder to my body for 12 hours; it's less likely to cause me injury that might prevent me from wearing a binder for as long as a few weeks or months, should it be serious enough. Obviously, the choice is yours, but also consider that most guys aren't 300% flat either; for reference, I've attached a side pic of myself, 2 months on T. I pass 100% of the time now without any facial hair or fat redistribution, and I'm by no means a tiny dude. Flat isn't everything!

I think you answered your own question: wear an old one that won't cause you pain. Just wear something like a flannel shirt or hoodie that visually aids in the appearance of "flatness" as opposed to a t-shirt or multiple t-shirts.

Wear an older one and find a way to layer your clothes to cover up anything you're not comfortable with. Even if I don't wear a binder, I can typically layer enough underneath a hoodie to get gendered correctly.

Thank you guys! You're probably right, I think I'm gonna wear and old one, at least in school

If it really doesn't make you comfortable to wear the older binder while at school, why don't you wear the new one and keep the older one in your bag, to switch to after you finish school, so that you will be much more comfortable when going to the event?

Or, if you can manage, definitely sounds like the older one would be a much better idea for a long wear time.

As the others have said, layering up is also useful, as well as wearing clothes like flannies that are patterned or highly graphic.