Why cant girls and Boys sit together in schools?

So In our country most of the school are either boys only or girls only there are alot of school where there are both boys and girls but even If thats the case girls usually get different sections or rooms than boys. AND even If they sit on the same class girls sit on one side and boys sit on another side.

so why is this?Why cant they just sit together? Its not like if the teacher turns away for 2 seconds they'd start making out.

I remember in Pre School I could sit next to girls but from class 1 our school got separated into boys wing and girls wing.