I want to find manga about two popular guys in their school (not boys school). They are bestfriend or something like that. Both of them are so cool and girls crazy for them. But behind their cool behaviour and friendship,actually both of them are a couple. They hide their relationship from others.

There's one manga that fits, except they don't hide their relationship.
Momoiro Junjou Danshi
It has a sequel, and I thought it was pretty decent, so try it if you like. 🙂

Umm, King and Prince? If you haven't already read it.

Thanks for the suggestion. Hope you can give more suggestion.