Cherry Teacher Sakura Naoki

tl;dr: A manga without any sort of plot but a very strong and consistent level of humor and art. 

This is a manga that relies entirely on it's art and humor. The main cast of characters is interesting and likable due to how amusing they are, and a few of the side characters you see a bit of are decent enough as well for the same reasons. But there really isn't any character development, relationship development, or plot at all, large or small scale.

Rather, the manga is just a series of really short amusing skits, mostly around 10 pages. Thankfully, these are all really amusing with great comedy and in general a really humorous tone and atmosphere, though one that's also generally pretty ecchi in nature. This is enhanced by the art being really good at visual comedy with lots of exaggerated expressions and such. However, even beyond that the art in general is excellent with a great style and quality with great framing resulting in lots of great panels and just in general flowing really well.

What's most disappointing about this manga is how it was axed as it felt like it could have kept up the solid level of quality for quite a while yet. The ending itself is kind of strange, in that because there was no plot there wasn't really any sort of plot to end but there's an attempt at giving it a solid ending anyway. Though I wouldn't go as far as saying the manga succeeded in that it probably was the best way to end it on short notice considering the context, wherein despite the manga not having any sort of plot at all, it still managed to invoke some level of emotional reaction from me. 

Short:
If you like the principal from Prison School, you'll like Cherry Teacher. It's a great ecchi comedy.

Long:
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.