The School have a duty of care to feed children. If the account isn't topped up (the same as a parent not giving the kids cash or the kid loosing cash) the School will feed the child regardless. As an aside kids can enter coins/notes onto their accounts at a box on the wall.
The only downside I see to this system is the smaller kids may get fatter as they are not running away from bigger kids trying to steal their coins. Man I did a lot of running at School