Many times, flirting is welcome attention. It goes both ways, and both people enjoy it. Flirting doesn't make you feel bad. It's perfectly fine for kids to flirt with each other in school.
Flirting is OK because it can be between two classmates who are both doing the flirting, and no one is being dishonored. But it is not OK for adults to flirt with kids. In fact, it is illegal for adults to flirt with kids.