Hi! I'm looking for a manga about average high school (i think?) girl who changes to the worse because of her admiration toward people who are cool (not cool, maybe fashionable and can do what ever they want). So in the beginning she took her study seriously and was wearing glasses her hair was black too. After the change her hair became blonde and her skin tan. The most important part is that she began taking drugs and drinking. Also, I'm positive that there was some sexual scenes.

[spoiler] Plus I think it was sad ending.

I'm not sure about some details, but I think that the manga is recent (maybe it was published one or two years ago, or maybe even before).

It will be really great if someone can help!

Might be a bit of a stretch, but perhaps Emergence? It is a famous hentai that seems to match your description.