1. They expose your feet to bacteria, viral, and fungal infections. Any time your feet get particularly filthy (i.e., any time you wear your flip-flops in public), they're likely covered in some nasty things likes Staphylococcus, a bacteria that can irritate the skin on your foot in the best case or lead to amputation in the worst-case scenarios. (It depends on whether you have open wounds, like microwounds from exfoliation during a recent pedicure, or actual cuts, and your state of health when you pick up the bacteria
Unfortunately, if the school that you are going into is bad enough to allow any bacteria deadly enough to effect your body in such a way, they school is more at fault. The excuses you are using is more so that schools are not required to make sure the bacteria is cleaned from school grounds. Flip Flops are allowed within stores, and many other areas, so why would schools have a problem with it when no-one else does?
They destroy your heels. Because your heels hit the ground with more force when there's nothing but a measly piece of foam separating your foot from the ground, walking in flip-flops accentuates the heel-strike impact, which could end up causing pain " especially if you stand or walk in them for extended periods of time.