I read this a quite long time ago, so my details are vague. The art style is rather realistic and great tbh so i assume its a seinen. This manga is about a bunch of middle school boys (14 at maximum i think) wearing some type of uniform. Similar to average japanese black middle school uniform but their style kinda of reminds me german soldier attire (especially with the hats) like its some nāžī parody?? This kids have a big buildings to themselves and they do weird stuff like kidnapping a girl, and there is also a huge robot with conscientious but these parts are very vague i barely remember them. This fake nazzi kiddos are certainly not kidding around. There were lots of violence and murder as i remember. One rememberable moment was this; the leader is a boy with glasses and he is rather vicious and idealistic. He has a sexual relationship with a pretty boy. But this is a secret from others. One of the scenes, pretty boy is upset because his puberty is starting and he is scared of losing his feminine qualities which other boys like him for (especially the leader). The leader is aware of this physical changes and wants to cut his ties with the pretty boy. Since everyone there are juvinile boys close to adolescence, they wanna see some girls... I think thats why they kidnap a girl. Also, Leader is annoying and most boys hate him.

Litchi Hikari Club or Bokura no Hikari Club (prequel)
While reading your description, this title and cover immediately popped up in my head. Never actually read it, so it's kind of a wild guess.

Omg thank you!!! You are a genius lmao