Is she having a tantrum because she does not want to go to school? If so, I would make sure she goes to school on time, to let her know that throwing a tantrum does not change the fact she has to go and on time.

She really knows how to push your buttons and test you, but YOU are the parent, and she has to learn that. Get down even with her - looking her in the eyes, tell her you can see that she is upset, but that going to school is something she must do, and tell her if she uses words, rather than tantrums, you can help her better. I vote strongly for her going to school on time, even if she does not apologize (for whatever she said or did). I would also set up some strong "consequences" for actions such as tantrums. If she has one before school, she does not get a snack or TV or whatever she really really likes after school. Stay consistent with her now, it will help her learn boundaries now, to prevent horrible problems later ! Good luck !!!

P.S. Have your tantrum after she is at school ! LOL !!!