I went to a Dutch high school that used Google Accounts for email, and they once caught some students "cheating" on a group project (i.e. collaborating in larger groups than they were meant to collaborate in, I guess) via email. This made me suspect that the admins could read our school email (which people also used to talk about various other stuff, which I guess was unwise). I don't know if that was actually how they found out, but it made me very conscious of email privacy (or lack thereof).
Now I work at ProtonMail, so go figure.