Manga to read for school

Me and my friend have a chance to read any book we want for school. We both love anime (AoT, Tokyo Ghoul, Code Geass) and would like to read a manga.  Obviously since this is for school it needs to be a longer one though. Can we get some suggestions please and thank you!

One Piece, if your looking for a really long one.

EDIT: If you would like I can post all of the mangas that I have read and enjoyed. Also, you should specify what types of manga you are looking for.

Prison School, it's only appropriate (͠≖ ͜ʖ͠≖).

  • Ana Satsujin -  it's about a suicidal teenager who finds his reason to live when he falls in love with his neighbor, except "PLOT TWIST" his neighbor is a Serial Murderer. this manga has alot of "HOLY SHIT it's about to go down moments"
  • xxxHolic - it's about a boy who has spirits that follow him on a regular basis, he meets a witch named Yuuko, who get's rid of the spirits that follow him in exchange that he becomes an employee in her shop she teaches him about the supernatural and that nothing is truly coincidental,
  • Sora No Otoshimono - it's about a boy named Tomoki, who wishes to live a normal life, however, he has nightmares of an angel and continues to wake up and cry every morning, his normal life changes and turns upside down, when an Angeloid, named Ikaros, comes down from the heavens, and begins to call him master. [if you enjoy comedies, with mild perverted moments i recommend this]
  • Elfen Lied - the story revolves around dicloniuses (humans that have been mutated to have telekinetic powers) these beings are violent and vary from person to person, the main diclonius Lucy develops an alter ego after taking a massive wound to the head after trying to escape the scientists that mutated her, Kouta the protagonist, finds her and names her Nyuu, since her vocabulary is limited and she cannot speak her name properly due to the head wound, and the story begins when he decides to take her into his home.
  • Nozoki Ana- This is probably the most story driven manga i've read in a long while, and honestly, fuck it i'll admit it, i shed a tear after finishing this manga. this story reolves around tatsuhiko an art student who, moved to Tokyo to attend an art school and start his new life. In his new room, there’s a small hole in the wall. At first he can see nothing through the small hole, but one night, through the peeping hole, he saw a girl. and the story begins, she finds out he saw her and she black mails him. [Word of advice, this manga does have a few bits of "Adult" content if that is something you don't like to read in your mangas, well honestly, there are some borderline hentai scenes in this manga) i know this comment is kinda getting old already but i'll say it "its a WAAAAAAAAAAAY better love story than twilight."

Sorry if i couldn't help or if you don't like these,  my preference in manga, is all over the place when it comes to genres.

Also I SECOND "Prison School" i enjoyed reading it, it's still ongoing as well.