I would look into a few things here- or rather I would encourage your dd to look into a few things here (seeing that she is almost an adult and should be taking responsibility for her life).
i) if she is up until the early mornings studying could it be that she is simply not working efficiently enough? I know it's hard work in Sixth Form (have a dd with health issues in Yr 12) but putting in more hours is not always the answer. Might it be an idea for her to see her tutor and ask for tips on study techniques?
ii) if she has bad periods she needs to bring in a medical letter from her GP explaining the situation. She also needs to speak to teachers about what she can do to minimise the impact. My dd has probably had a similar absence due to a bad back; the school know about it, and dd will email the relevent teachers about work done at home if she is unable to come in.
iii) she needs to treat attendance as she would do if she had a job- it is not for her to say that mentoring sessions are useless. If that is part of her day then it;s part of her day. An employer would not accept an employee
iv) plenty of jobs have a non-food and drink policy while you're working. When did you last see a cashier at a check-out or a librarian at her desk drink from a bottle of Oasis? They don't want messy sticky stuff in proximity to their goods. Simple as. In the workplace she would have to stick to the rules.
v) as for expecting to treat the time after school as her leisure time- if that were the case, school hours would have to be far, far longer. Most of her friends will be spending time studying in the evenings and during the holidays and she is unlikely to be able to keep up with her peers if she doesn't. Even if she is getting A*'s for every piece of work she submits- those exams are still round the corner/