Get her up earlier.
A cup of cocoa made with milk might appeal.
Otherwise:
Small yogurt
Small fruit smoothie or experiment with yogurt plus fruit, etc
Protein shake
String cheese
Small breakfast muffin - egg, cheese, diced ham, mashed broccoli (bake, freeze, reheat in microwave)
Precooked sausages dipped in whatever she prefers
Fish fingers
Frozen pancakes or waffles with jam or syrup
Cheese toastie
Leftover pizza
Don't get hung up on sugar.
Chat and listen -
Does she feel anxious in the mornings?
Is her lateness caused by misgivings about school?
Does she feel a knot in her stomach?
Is the school over strict?
Is she afraid of getting in trouble?
Why no backpacks?
Are uniform requirements very strict?
Does she like her classmates?
Anyone she doesn't like - why?
Any teacher she doesn't like - why?
What is the classroom experience like for her?
What is the playground experience like?
If she's anxious, she could try some centering exercises where she identifies or names her feelings, maybe breathes mindfully, maybe does some small physical motions (stretching, pushing against a wall, releasing tension in her neck by rolling her head around), then on to focus on her physical reality, and engaging with the job at hand. It's important to acknowledge anxiety, sit with it, get through it, and not try to deal with it by an externalised response.