As the others have said definitely speak to the teacher.
Unfortunately it's actually quite common in primary school for food to go missing, we have a case nearly every year. The amount of kids that are given almost nothing, in some cases I've seen firsthand, a cupcake to last the day, these poor kids feel they have no choice. Usually the teachers are aware of which students to watch and at DD's school there are actually a few kids who are provided with meals most days.
It's actually very upsetting that their are kids out there as young as 4-5 years old resorting to theft just to put food in their poor little tummies.