Mirroring internal screen

I have a notebook that I connect to my school beamer with a HDMI cable. In Trinity Control Center I can see both my laptop screen and the HDMI screen. I cannot use it if I don't "extend the desktop" to it, but then all my apps open stretched overt the two and control is very difficult. I can set the external screen to the same resolution as the internal, but there is no option to just mirror the internal screen on the external. Am I missing some black magic? Thierry