I for some reason think it's so cute if a boy falls in love with a girl that's like years older than him. And I don't mean just 1 year, I mean a BIG difference. Like he's in elementary school while she's in high school. She's shy and introverted, and he's reckless and brave ^_^ But he has to be really mature for his age.

You mean stuff like Merupuri?

Let's see:
- Rising Impact - not much in terms of romance until the ending

- Dear Mine
- Please Save My Earth
- Soredemo Sekai wa Utsukushii
- Kumiko & Shingo - largly untranslated
- Otoyomegatari - while the characters don't fit your request 100%, it's a very good manga and definitelly worth a shot

- Bitou Lollipop - the age difference isn't that big

Seconding Dear Mine and Otoyomegatari (one of my favorite mangas, gah, so cute). I know you don't want one year age gap, but Choco to Mint to fits want you are looking for (reckeless boy x introverted girl) and in the beginning she thinks he's a elementary student, but they're both in junior high. You could give if you don't find anything...

GAME OVER (Mizutani Fuka)?

Unbalance x 2. Teacher student relationship