They should eat breakfast before going to school and attend class. If they don't eat breakfast, their brains cannot work well and they don't get energy to attend class. Their brains work by the food they eat. You should let them get their brains work in school obviously because school is a place to learn.
Breaking the fast is obviously and important part of our metabolic day however if what children are being fed before going to school is chocolate, fizzy juice or crisps then the overall health will be poorer. High sugar cereals etc are just as bad as these may cause a sugar spike and a sugar drop so by the time they get into class they are actually functioning at lower level than if they had not had anything at all