I have taught at both middle school and high school, and the differences are fairly minor. High school students are more versed in the process of "doing school." They will quickly determine exactly how much work they have to do to obtain the grade that they want. They understand that failed courses will need to be made up in some way. High school students have so many more outside obligations and interests that you have to work harder to earn their attention. It is not unusual for high school students to use work time in one course to do the work for their next class. Overall, whatever grade level you teach, you must be honest with your students. Don't threaten when you won't follow through. Don't allow yourself to be backed into a corner -- decide what is most important to you and don't compromise those core values. Most importantly, show the students that you care about them and that you understand that they are complex people of importance.