I think teachers get more abuse than cashiers and servers. Stores and restaurants are occasionally willing to kick out their most obnoxious customers. Also, since what those businesses provide is more straightforward and clearly-defined, it is easier to give the customer what they asked for and send them on their way. Moreover, cashiers and servers generally don't have to deal with the same customer (or their children) day after day for an extended period of time, no matter what that customer says or does.