If she's worried about punctuatlity ,help her to think through what time she needs to leave in order to get there about 10 min early.
Then talk through some of the things that might happen to delay her - bus/train issues, cycling - can't find a place to lock her bike or it's further away than expected, lock is lost or doesn't work properly, etc; walking - distracted by others, tempted to stop at shop, getting a bit lost, etc; leaving the house - forgetting something and needing to go back; not finding something important that she needs etc, problems locking up. And build in some buffer time for things like that delaying her, so that she's not worried about being late if it happens.
Finding solutions for as many of the above as possible - packing bag the night before, finding all uniform/keys, checking bus/trains/bike tyres, etc. Or in advance - finding out alternative routes or timetables, or what to do in case something happens.
When you've decided on a leaving time, see if she is able to plan herself to get ready for that time (maybe on a weekend). If not, start working backwards with her to plan how long various steps of getting ready need. You can use reminders on her phone or alexa or whatever to prompt however many stages are needed, including the final 'you should be leaving the house by now' one.
You can adjust how much (if any) support she needs, once you see if there are any problems and what they are in her getting herself there on time.
Also find out what consequence there is for lateness; if it's issues beyond her control like buses/trains, you can phone in to explain. If it's detention, that might make her really anxious - often it gets easier once they've had one, as then they know what to expect, and that it's annoying and boring, but not actually awful, and that eases their mind a little so that they don't panic about being late.