If it's a school trip, I"ll also assume that transportation costs are covered, as well as admission to any sights or attractions you plan to visit. So I assume it's just shopping/spending money? But it's not entirely clear what "everything else" is - because you'll need a lot more if you need to pay to get around and for the things you're going to do. And what you need will depend on what you do in that case.
I'd suggest giving yourself around $30-50 a day for souvenirs, snacks, and other things that you might need/want to purchase. You probably don't need that much, depending on what exactly is already covered. But admission to museums, observation decks, and other attractions can be $20-50. Broadway tickets are generally $100 and up (some can be way more than that). You can find cheap souvenirs, or you can find high-end shopping. I imagine you'll be sticking to the cheaper end of things for that though.