Well, I'm a little ashamed that I forgot it in the first place, but I've become just a bit desperate. From what I recall...

It's a multi-chapter one, and has two main pairings. The first features a boy and his teacher, who isn't exactly the nicest guy. (Whatever, I don't like the pairing anyway)...

The second pairing is what I really want to read. The boy's older brother, (who has a serious brother-complex) ends up with Boy #1's best friend.

The later chapters skip ahead in time.

Congratulations to you if you have any idea what I'm talking about.

Thanks.

Sounds a bit like "Welcome to the Chemistry Lab" (a.k.a. Kagakushitsu e Douzo) by Honjou Rie, though there are other oneshots along with those two stories. I think the 2nd and 3rd volumes might continue those two stories, but I haven't read them yet so I'm not sure.

Oh, that's it! Thanks!