Most schools have very thin walls on the floors themselves.
First off, the best bet is to connect a router configured as an access point to an unused Ethernet port. This enables wireless clients to 'jump onto' the wired network at that AP. Then walk around the floor, get a feel for what interferes with the signal and places it is weak.
Then, find a spot right at the edge of that router's reach and put another AP up. Depending on the size of the building, 2 may be perfectly fine. Three in a triangle pattern would almost certainly work, again depending on floor space.
Guest networks come built in to essentially all Buffalo routers, if you don't want the students using it for piracy and other distasteful things, lock it down or give out passwords.
That's it. Wireless network in a nutshell. There are some configuration details but that is most certainly a possible and simple setup.