A server is a computer that provides data to other computers. In this case, I'm using it to mean that someone on the internet-at-large would want to access your computers without you having to initiate the connection, as if you ran a web server or a mail server or something like that. If you have no need for anyone to access your computers externally, then you have much less need for a static IP address. Keep in mind that external access includes remote administering.

Your scenario doesn't say anything about incoming traffic, and none of what you said indicates that you need a static IP, which is not to say that you don't need one; I just don't have any data that indicates you do at the moment.

Wireless routers include NAT. They do an okay job of it. If you'd be happy with that, then you don't need your ISP to provide NAT service. However, 15 remote sites sounds like a lot of computers. (Hell, "college" sounds like a lot of computers.) I don't think that a generic wireless router will be powerful enough.

You need NAT if you have more computers than you have IP addresses assigned by the ISP. There are other reasons you might want it, but you definitely need it in that case.