You have several options. You can use Insert From File to insert slides from
another file. Depending on the version of PowerPoint you are using, this
might or might not preserve the design template (if the students picked their
own templates).
Another alternative is to create a master slide show. This could contain a
title slide and a menu slide that has buttons that link to each of the
students' presentations. Just get all the presentations into one folder on
the same computer and create the links. You could even put an End Show button
on slide of your students' presentations so when that button is clicked, it
will take you back to the menu.
Finally, there is something called a playlist that will play several shows
in order, but I have never used it so I don't know much about it.
The bottom line is that you have several options, and there is no need to
have the students all work on the same computer as long as you can eventually
get their individual shows to one computer.
Oh, one more thought. You might consider having them work off of a template.
You could create a template that includes a common background (or not) and
includes the End Show button.
Good luck. Let us know if you have any further questions, and if you are
still frustrated (my job is to frustrate teachers--well, at least, that is
part of my job).
--David
David Marcovitz
Author of _Powerful PowerPoint for Educators_
http://www.loyola.edu/education/PowerfulPowerPoint/