Classroom projectors have been having issues displaying laptops. There are two possible solutions depending on if the laptop in question is a Mac or PC. 

PC: If a PC laptop will not display on the projector (and the projector is working for the desktop computers), try hitting the Windows button (lower left-hand side of keyboard) and P at the same time. You should be presented with a number of different display options, such as Duplicate or Extended. Use the Extended option. This should solve the issue. 

Mac: If a Mac laptop is not displaying on the projector, it is likely because the wrong type of adaptor is being used. The HDMI to VGA adaptors will not work to project a Mac. Use the Mini Display to VGA adaptor available at the 3rd floor service desk to connect to the projector. You may also need to make sure that mirroring options have been chosen on the Mac (in System Preferences-->Display).