How would the life of a high school student be different under a totalitarian government than it is under a democracy?
This depends to some extent on the beliefs held by the people running the totalitarian government.
The life of a high school student would most likely be much more oriented towards being indoctrinated in the beliefs of the government. For example, in Nazi Germany, there was the Hitler Youth and pretty much everyone belonged to it so they could be taught how to behave as good Nazis. No matter what the government believed, it would try to make you believe that as well.
Most likely, your out-of-school life would be much less free. This depends some on what the government's ideology was. However, a totalitarian state would not be likely to tolerate high school student rebellion through such things as wild parties or violating curfew or petty vandalism. It is likely that such a government would be very strict on teens. This would apply at school as well. It is unlikely that the government would allow students to talk back to teachers or to otherwise misbehave at school.