How do I teach my 3 year old that she should not bite other kids in school?

First thing you have to do is determine why she engages in the behaviour. That's when you can solve the problem. Does she do it when she's upset or frustrated? If so, then teach her how to express herself (explain why she's upset) and managing her frustration (counting to 10 or playing until her anger dissipates). If she does it because she's excited or just for fun, then: give her other options to express her excitement; and other fun activities that aren't hurtful.