Debian GNU/Linux in a lab of about 50 computers.Is there any possiblities of doing something like creating a image in one hard disk and installing it from that instead of installing the base system in each system individually (something like gHost images for the M$ OS)....

If all the machines are identical in their hardware configuration, then you can install the operating system on the hard disk of one of them, and use GNU Parted to copy the entire filesystems from the original machine to the others.

# parted (parted) help cp

Or consult the manual at http://www.gnu.org/software/parted/manual/

The advantage with Parted is that it is a part of all the GNU/Linux distributions that I have used-- Debian, Ubuntu, and Fedora.

Cheers, Debarshi