Responses
The description reminds me of Araburu Kisetsu no Otomedomo yo/O Maidens in Your Savage Season
http://www.mangago.me/read-manga/araburu_kisetsu_no_otomedomo_yo/
is it O Maidens In Your Savage Season?
OMG YEEESS. It's Araburu Kisetsu!! Thanks you both!!
Hi! Some day I took a quick look at a manga in a store. I don't remember the title, neither the plot. I just remember a scene, more or less. A male teenager jerking himself off (in his room I think?) and then a girl comes in and sees him. The manga was about the discovery / development of sexuality in adolescents. It wasn't hentai XD There were more than one protagonist I think.
It must be difficult to know the manga I'm talking about due to the bad descriptions that I have given but if you know of any similr manga at least, I will appreciate it!