Regardless of the speaker, the words "shut up" are considered to be impolite. No one likes to hear those words. If a child were to tell an adult to "shut up," I suspect any adult would see this action as defiance.
If a child one time were to tell an adult to "shut up" at school, I suspect a mini-conference would be held immediately with the child in order to discuss the inappropriate behavior. However, if this were said more than once to an adult, or with other adults in close proximity, or to a group of adults, I would expect my child to be taken to a private area for the situation to be discussed more fully.
If this were a one-time event, private conferencing would be probably be sufficient. However, if this was a repetitive behavior or a response combined with on-going misbehavior, I would expect a child to have a "time-out." Depending on the frequency and/or the severity of the misbehavior, a child could find himself suspended from school for a day. In spite of a conference or a suspension, the biggest concern would be to identify why a child were acting out.
Our role as adults is not to criticize, belittle, or punish children. Our role as adults is to guide them towards positive behavior when one fails to meet appropriate standards. Unfortunately, a "time-out" is sometimes needed for a child to think through his inappropriate behavior.