Does next year mean next academic year - so, in just a few months time?
Does she need the money? (as in, are you able to and willing to support her?)
I got my first PT job at 13.
In the summer season I often worked 60 hours a week, off season maybe 20.
At 16 I worked no less 20 hours a week through college and then uni, full time in the summer. As many hours as I could get - I had to leave home at 16 and was self funding at uni (post grants, pre tuition fees)
In my placement year at uni I couldn't afford to lose my PT job for the final uni year, so I did a 6 day week all that year.
With the exception of 6 weeks inter-railing between uni and grad job, I've worked a LOT of hours for 30 years.
I enjoyed most of my work. It helped me get further work opportunities. I didn't have a choice anyway - but it was fine, you do what you have to do.
You know what though? Two shifts a week is enough to learn to be on time, to report to someone, to learn new things, to work with others, to feel the value of money (that moment when the £20 top represents having been on your feet dealing with shitty customers for 4 hours!).
I say, having worked so much - she has a lifetime of work ahead of her. Let her enjoy having leisure time, even if that's seemingly rubbish stuff like hanging out with mates - not worthy or personally / career developing volunteer work.
If I could afford it, I'd let her choose, for the summer.
A full year and I'd want more of the worthy stuff 
It would depend for me if she needed to be 'taught' about responsibility and money, or if she thought I'd be funding a Primari habit.
You may well find she increases her shifts without being pushed once she starts to enjoy the money and realises she hasn't much else to do.