Linux is a completely free, public-domain version of Unix for PC's.
It is a FULL Unix  --  Unix kernel, shells, compilers, editors, 
X11 window system, etc.  --  everything.  There is a tremendous 
variety of software available to add to it, all free.  It can
coexist with DOS.  It also allows you to use your PC as a remote
X11 terminal to other machines via modem.

Not only does Linux provide you with great convenience, allowing
you to do your homework at home (without tying up your phone line!),
but also is an excellent chance for you to learn things about
computer systems which you will never learn in school.

The best (though definitely not the only) way to obtain and install
Linux is through CD-ROMs.  I recommend the Red Hat distribution.

Installation, etc., is well documented in the distributions.  And
again, the installation process itself will teach you some valuable 
real-world knowledge which you will not get from school.