The first question I have is do you really mean subnetting or do you just want to hook all of your devices up to the same network? If you truly want to subnet - that is have different sets of PCs on different networks that are isolated from other sets of PCs - switches alone won't do the job. You'll need routers in there as well. (And combination switch/routers are common.) If you just need to fan out the network connection to all your office PCs, switches will do the job fine.
The term "subnetting" roughly means you want to take the network IP addresses you were assigned and break them up into groups of separate networks - each with a contiguous portion of the original IP addresses. You may do this for a number of reasons. One typical one is the servers you have generate a lot of network traffic between themselves (such as a database server on one machine, an application server on another machine, and a web server on yet another machine). If you isolate these machines on their own subnetwork, that traffic doesn't bog down your other office machines that might just be getting email and sharing the occasional file. (The server machines could also take further advantage of the separate, faster [gigabit] network) while the office machines are using standard 100 Mbps Ethernet.
You may also have meant that you want the servers reachable directly from the Internet (e.g., a web server), but you'd like to isolate the office PCs on an internal LAN that can't been seen directly from the Internet, but still have access to it for web browsing and email. For that, routers are definitely required.
In order to keep the server traffic isolated from the office traffic, you need a "traffic cop" or guard. That's the job of a router. The router connects (at least) two networks together. It looks at the destinations of all data traffic on both networks and decides when a message sent on one network need to be delivered on the other network it is connected to. In other words, it routes messages when necessary. If one of the office machines browses to your web server, the router will recognize that message needs to be routed from one network to another. It will also recognize that the response coming back from the web server in another message needs to be routed back to the office LAN. (I hope I'm not being to juvenile for you here. I'm just trying to make sure it's understandable.)
If your goal really is just to get all of the PCs talking on the same network, you just have a fan out problem to solve. The fiber connects each of the buildings, but it's really just a fancy ethernet cable of sorts - with very low loss. If you connect the switches to the fiber interfaces and the PC/servers to the switches, you may well be good to go. Because you are not routing, all traffic from any PC in any building will be seen by any other server or PC in one of the other buildings. For 70 PCs doing office work, that may be fine. If your servers are really servers with lots of traffic between them, you probably want subnets to isolate the traffic.
If you describe further what your goals are, we'll be able to give you a more useful answer.