Ahhh, but was he told? There seems to be a question on that per the article (underlining is mine):
Director Sam McCollum said his son's account was also in arrears, but he only learned of it when he checked online, through Nutrition Group's Web site, mylunchmoney.com. His son was never notified.
Dr. Jerome Bartley, district superintendent, said the policy is to not have charges whatsoever at the middle and senior high school. Workers are supposed to remind a student that his or her account is low whenever the balance drops below $5, and they are to refuse service when it dwindles to zero.
Bartley said he will check with the cafeteria workers to make sure they are following the policy.
Not being there ourselves, we don't know what actually occurred on or before the incident.
At the risk of sounding like a broken record I'll say it again. If there is a website where the parents or kid can access their lunch acct, and check their balance there is absolutely no excuse for either of them to not know, regardless if they were told by the cashier/teacher/principal or not. Its called your responsibilty.