You can float on your back, but if you have low body fat it's not so easy, at least in fresh water. If there's more than a handful of students all trying to do that in a smaller pool, that doesn't help either. Floating vertical 'waterpolo' or 'drownproofing' or whatever-you-call-it style is usually a better option.
If you don't swim too well, go sidestroke or breastrstroke. Either lets you see where you're going, it's easier to lift your head above water to breathe, and they both utilize the much bigger leg muscles.