As I'm in the middle of my Sket Dance and Gintama marathons (I've been putting them off for quite a while now) I feel I need to read another still funny but less funny manga. All this comedy is burning me out. I'm looking for something realistic, endearing, on the long side. It can have dark moments and be a bit tragic at times since I'm more of a seinen guy, and I'm also a big fan of slice of life.
My request comes to be because I'm simply tired of waiting for other mangas I'm reading, so if it's finished that would be a big plus.
So what are the best high school manga you could recommend?

Try Boys on The Run

While it does sound interesting, I don't see how high school fits into the story. I'll give it a go. Any other suggestions?

um... unfortunately a lot of my high school selections are shoujos... some of the less romancy ones though are:

My highest recommendation: Again!!
a social outcast who looks like a delinquent finds himself regretting never joining his school's the Oendan (it's like very masculine cheer-leading). In the whole of his high school career he hasn't had a single friend; he hears his classmates wonder why he even comes, since no one would miss him. On the day of his graduation ceremony, after falling down the stairs, he wakes up on the start of school and has a chance to fix all of his regrets.
~I think it has a good balance of funny and serious.

Others:
Iris Zero - a lot of this takes place at school and pertains to classmates, this also has a lot of angst / bullying

Mahouka Koukou no Rettousei - the ultimate magic high school series~! this doesn't utilize the high school genre enough I think... most of the arcs have to do with magic-high school specific things...

Yanki-kun to Megane-chan - I only watched a little bit of Sket Dance, but it reminded me of this series...it's totally not dark though

You could also try Okujouhime. It doesn't have much funny moments but it's endearing 🙂

Yes, you're worthy!

Sakamichi no Apollon and Ciguatera

@steaiii I've read Yanki-kun to megane-chan already and it was good, it didn't have too much on romance though. I'm not too big on fantasy or magic, prefer my series to be more on the realistic side. Again!! is good and I've been waiting for translation to end.

@jonytep sounds good, I'll give it a go.

@Jyeims both sound interesting.

Thank you all so far! More suggestions are always welcome!

+1 for Sakamichi no Apollon and Ciguatera.

These are popular but your lists aren't public so I'd give a try:
Onani Master Kurosawa
Angel Densetsu
Ore Monogatari!!
Gin no Saji
Horimiya
Kyou kara Ore wa!! and Joshikousei (too much comedy though lol)

Uh.... so high school + romance + preferably Seinen-ish?

Maybe Nazo no Kanojo X? Once you get past the initial weirdness, it's an interesting series.

I was about to recommend Assassination Classroom (if you don't read it already) and perhaps Alyosha!, though there isn't much romance in those.

If you don't mind shoujo of various sorts:
Warau Kanokosama (not much romance, but the main girl is hilarious)
Ore-sama Teacher (not much romance, but the main girl is hilarious, a bit like Yanki-kun to Megane-chan)
Cherry Boy, That Girl (lol)

Well, maybe you'll at least find one of them interesting.

Just finished Ciguatera, very, very nice.

@bwfas I've been reading Gin no Saji (a 10 for me) and Horimiya. Known about Onani Master Kurosawa for quite a while but the description didn't really appeal to me. Read Angel Densetsu about 3 times now I think, it's a perfect 10 for me. Ore Monogatari!! is the story that appeals the most to me because a couple of years ago I thought of writing a story along those lines, I'll definately follow it.

@Mizura Mysterious Girlfriend X was actually one of the first 3 mangas that got me started, such a shame knowing it'll end this month, but I suppose the story was dragging. I'm also reading Assassination Classroom and I've finished Warau Kanokosama a long time ago, I've been reading the sequel too. I'll look into all your suggestions.
Thanks for taking the time to post, and if there are any more suggestions please let me know, as you can see I'm a fast reader.

Thirding Sakamichi no Apollon good match for the " realistic, endearing, on the long side" request.

Quote from nail80

Just finished Ciguatera, very, very nice.

Huh that was fast, maybe try Hoshi no Furu Machi then? You may like it too, but it's a lot more on the romantic and sweet side than the realistically and dark one.