I’m using my school computer and I’m trying to boot using a live Ubuntu cd

SO when my computer starts up I press F9, which brings me to to boot screen It asks me if I want to boot from the hard drive or optical disc drive, which I then click. Well it seems to load for a minute but then the screen goes black and the only thing I see is what looks like a blank command prompt. It then goes back to booting Windows 7. Need help, now.