Copyright and Fair Use LR

I do not understand why it is okay to make back-up copies (well, archival copies) of a CD-ROM program, but it is not okay for a teacher to copy a CD-ROM so that all of their students can use it. To me, it is the same action; a CD-ROM is being copied. Why is it okay to have back-up copies? Is it okay because they are not all being used at once? I do not understand it! Personally, I feel that if you paid for the CD-ROM that you should be able to copy it if you wish. I hope that I will understand copyright laws a little better in the future so that I do not get myself into any trouble with the law.