block certain dangerous commands?

In my High school there are few computer having Ubuntu 14.04 installed and many students know the Root password, how can I block/disable certain dangerous commands like :- rm -rf / rm -rf * :(){:|:&};: to avoid Data loss ?

Impossible. Absolutely impossible. If they know the admin password they can do a-n-y-t-h-i-n-g and can always revert any change you make to the system.

If you are the admin of those system you need to re-install those computers. There is no telling what those students did to those systems (for all you know they installed a keylogger, a mailserver or some dubious script and know your password seconds after you changed it) and create an admin account with a secure password.

Cannot change root password as many students and teacher use it !!!

Yes you can and yes you should. There should be 1 or 2 people that know the admin password. Other people can have elevated access but they still do not need the admin password.