Taking your school for an example:
If your school wanted to buy some software to use on EVERY computer in the school, there's a few ways they could do it:
1. Buy the software multiple times, giving each computer its own copy of the software to install and run. Eg: You buy 300 copies of Photoshop and install it off a DIFFERENT cd on each computer (each computer would have a different CD-KEY).
This is simply buying a 'Single User License' for each computer.
Social/Ethical Issues: Its expensive - VERY expensive, especially for 300 copies of PS...
You can keep a backup of each CD. So you could have 300 backup cds...
You CANNOT install from one CD ono multiple computers however. If your school chose to do it this way, they MUST buy a separate copy of the software for each computer is was going on. (That is why it is expensive.)
So software companies provide a number of options for people wanting multiple copies of the software.
2. Site License:
You buy one CD, and get a CD-KEY that entitles you to install the software on ANY computer that is a part of the site that it has been bought for. Whilst this is expensive, it is MUCH cheaper than buying a separate copy for each computer.
S/E Issues: You CANNOT use this software outside of the site. Technically, if you need to take a computer off the site for some reason and use it, the software must be DELETED from that HD first!
3. Multi-User License:
This is similar to a site license, but it specifies how many computers you are able to install the software on. So you might buy the license to allow for 100 installations of Photoshop.
S/E Issues: You CANNOT install the software on more than 100 different computers. If you need to, you either have to buy more Multi-User Licenses, or DELETE it from another HD first.
But sometimes its impractical to install software on EVERY computer. If you have a network, then you wouldnt need to either, as the software can be run off the network.
4. Concurrent Use License:
This allows only a certain number of people to access the software concurrently.
S/E Issue: If you have a greater number of people accessing the software at once, this is illegal. You would need a way of regulating this.
You CANNOT have more than the specified number of people using it at once.
5. Domain License:
This allows ANY number of people to access the software over a network.
S/E Issues: Depending on the specific license, this would say how many servers you can install it on, how many sites etc.
You should just read the license agreements that come with some of your software to find out a few more 'picky' social/ethical issues...