Difficult situation. Be firm - not passive or aggressive. Talk to her well before she has to leave about why she resists - do not apologize for sending her to school. State simply and plainly that she has to go to school. If she doesn't get ready then you simply pick up her things and toss them in a bag and take her out the door as is. You cannot stop her from having a tantrum, I wouldn't give her any attention during a tantrum beyond reminding her that she's a big girl and her behavior is not appropriate or acceptable. Ignore the tantrum beyond doing what you need to do to get her to school.