I think leggings would be ok, as long as you wear something like a skirt (I'm sure they would be fine with a school skirt) or even trousers on top. I know you said you don't have any trousers but maybe on the first day you could talk to one of the staff members there and ask them if what you wear is appropriate and what ideas they have. Plus, they won't make a big deal out of it if what you wear is slightly different to their expectations. As long as you show them that you are the type of person they want to employ. Good luck anyway!