Can you lock her lunchbox somehow? Does her locker actually lock? If it doesn't, can you give her a lock to put on it (unless it is against school rules)? Evil Sami would like your daughter to fill her locker with marbles so the thief would be easily caught out. Or a camera that would take his/her picture the moment the locker is opened. Or some kind of non-toxic but obvious dye in food that would reveal the thief's identity...okay, I'm going off the rails here, so I'd better just stop it!
I agree that a missive to the teacher and copied to the principal is in order, as well. Perhaps something like this:
Dear Mr. Teachername,
My daughter, Superstudent, has told me that some items have been going missing from her lunch box for the past few days. Evidently someone has been going into her locker and removing these items while she is in class. I wonder if you are aware of any other students that may be having this problem?
Since this has become an issue for my daughter, I am requesting your help in coming up with a solution. I am concerned not only about the theft of her lunch items, but also that someone could easily put things in her locker, as well, that don't belong there. Please contact me at XXX to discuss the best way to ensure this does not happen again.
I look forward to hearing from you soon.