I agree with the other posters, send her to school with things she will eat. There's no point in giving her food she won't eat, it'll just go in the bin, secondary schools are not bothered about checking lunch boxes for healthy foods, the kids just get on and eat. If she's on school dinners then the queue's might be so long that she's not got enough time to eat, at the school my kids go to, lunch time is like a zoo in the cafeteria.
Have you spoken to her about what she's eating and drinking during the day? does PE fall n these off days? she may need extra calories, a banana can sort that out.