any good short anime series?

ERASED is good and not super long, only 12 episodes. Really good if you like mystery/thrillers.

I also enjoy Higurahi no Naku Koro Ni, which was previously mentioned, but it's not exactly short. If you count it by season, sure, but to get the entire story across just the first two seasons probably reaches about 50 episodes. Definitely worth looking into if you ever have the time in the future though, but be warned it's horror (in case that's not your thing).

If you like light, fluffy series with no serious substance, Wakako-zake and I Can't Understand What My Husband Is Saying are sweet and only three minutes a piece, makes it really easy to watch during breaks. And for a romantic comedy that's a normal 12-episode length, Kaguya: Love Is War.