All I remember is that the girl had black hair and I think her name started with an "M"?
The boy is one year younger than her, she's cycling to school and I think his bike is broken and he asks her for a ride or something, so she lets him ride her bike (with her on the back) to school -- if I remember correctly.
And he ends up liking her and I think she likes him back... that's all I remember!
I also remember some comments from her friends that she's very kind because she has younger siblings.

Thanks for the help finding this!

Toshishita no Otokonoko (HIRO Chihiro)

~4400 manga completed
~1700 ongoing...