It is rude for a start Kids need to be paying attention to the teacher in class, if they are eating not only are they more focused on that but other kids may be sitting back with their own mouths watering not paying attention either.
Kids these days seem to think they need to eat all the time. In the old days if a kid said 'I'm hungry' the parent would say wait for lunch or dinner and we didn't have the same obesity problems that we have now.
If they have no time to eat then we need to teach them more organisational skills.