Planning is key.
She has to have everything prepared the night before - bags packed and checked, outfit/hair clips etc chosen and NO changes alowed (unless it unexpectedly snows or something like that). You have to be firm and calm about that. She may try to kick off but stick to 'the rule'. She will soon find it fun to consider carefully and plan the night before. Can you plan some outfits and photograph them for her to select from? A bit like a fashion show board? She may enjoy that. Or find a school wit a uniform!
Realistically plan times for: getting up and to the loo, washing, dressing, breakfast, brush teeh, out etc and build in a sneaky 15 mins.
You need to keep calm and use subtle bribery. If she gets pocket money then you say to her 'you are a big girl now, so instead of getting it like a baby, now you will get it split into 5 packets of XXp. Every day you get to school on time you get the XXp. If you get there early, there's a bonus of 5p!'. Make it sound like a game. If she doesn't get homework just try a small amount of money - say 5 or 10p treat money per day she gets to school on time (a positive slant, so not 'for not being late').
You need to keep calm. The more worked up you get, the worse they act. They don't mean to, they aren't trying to upset you but its what they do! It is button-pressing at its most effective. We parents hate being late, but children really don't give a monkeys!
Don't react. If she is dawdling, remind her once that the XXp is dependant on her getting to school on time. You need to be super prepped yourself too though!
Make sure she gets enough sleep and is relaxed that she is fully prepped and ready to go in the mornings.
Good luck! We had this at its worst and now get to school early, have games and nice chats on the way and he gets to play with his pals before the bell. It took a little while but works well now. Ite key is keeping calm and almost blase - 'oh reading in the loo instead of brushing your teeth? What's that sound? Is that the sound of 50p jangling in my pocket as you won't get to school on time today?' Then its almost a game 'darn it! I thought you wouldn't make it today, thats another 50p I own you! Grrrrr!'
Maybe she has friends who you know get to school early for a chat/play and she wants to join in?