someone told me today that it could be because my graphics card is not as good as their ones, but im 99% sure that rendering just uses cpu????
Again, it depends on what your rendering. CAD and 3D images require a lot of video power, in most cases more than CPU. It's possible the computers at school have a Nvida Quadro or ATI FireGL cards which are desired for rendering which typical GeForce and Radeon cards fail miserably at.