So I'm trying really hard to find this manga. So it all happens in the school, there we have a main couple, boy + girl being friends and falling in love (? sorry, can't remember) but I'm most interests in their friends, a serious, down to earth girl being with sensei, confessing her love to him but then him rejecting her I think and there is this laid-back boy who starts to fall for her. I stopped reading this manga but I'm sure it should still be ongoing. Help me please! I think it starts with "B..."

Could also be Ao Haru Ride ...

"It's like love is a bonus in your life. Even without it, I'm just fine. I don't feel like pointlessly searching for it. You don't have to make yourself unhappy." (Ookami Shoujo to Kuro Ouji)

"Why does it rain?" - "To create drama." (Drops (Andou Yuki))

No, it's not that one but thanks, I will try that one too 🙂

Dear Diary maybe?

Sadly, it's not. But thank you!

The art style was hmmm, quite delicate.

I remember the girl from the main couple having short black hair and the other girl, having light, a bit longer hair. She confessed her feelings to sensei when he was about to leave somewhere I think.

Marmalade Boy? It has all that.

Still no, but thank you!

Found it! It's Bokura wa Itsumo 😀