If you like the principal from Prison School, you'll like Cherry Teacher. It's a great ecchi comedy.
Cherry Teacher is about a male teacher at an all-girls school, and the ensuing funny ecchi things that happen! Sounds horribly generic, huh? But unlike the hesitating, teasing panty shots of so many manga series (Yuragi-sou no Yuuna-san, for example), Cherry Teacher goes all in on the comedy and all in on the perversion. Every scene isn't just about the ecchi -- they're all hilarious.
In some sense, Cherry Teacher parodies everything about the ecchi genre. From stalkers to hot springs to peeping up girls' skirts at the bottom of
the stairs, Cherry Teacher doesn't take any stereotypical ecchi scene in the usual or expected way. It somehow always turns into a hilariously stupid joke (just like the assman principal of Prison School).
Update: Finished the rest of the series. It gets much lewder and stops being a parody at some points, but the jokes are still funny.