GOKUSEN THEME SONGS:
Gokusen 1 | V6 - Feel your breeze | INSERT : NONE?!
Gokusen 2 | D-51 - NO MORE CRY | INSERT : Kamenashi Kazuya - Kizuna
Gokusen 3 | Aqua Timez - Niji | INSERT : Takaki Yuya - Oretachi no Seishun
so yeah, a JE group can sing a theme song.
yup, MatsuJun was lead of G1, Akame for G2, & Miura Haruma/Takaki Yuya for G3.
& don't forget the supporting cast too. they're big names! Oguri Shun, Narimiya Hiroki, etc.