Can a teacher force you to open your email in school?

if you are at school and on the schools network then they can probably do whatever they want. There should be some sort of school policy with regards to using school resources for things not relating to education. Now my guess is no teacher could ever get a warrant for anything so that was probably a lie. If the student was cheating and the email that the teacher looked at proved it then i would say your friend is toast no matter what.