Depending on your setup, there are a number of different options available, each of which have pros and cons associated with them. In the coming years there will also be a technology known as ADSL , which is quite similar to today's leased line, but offering a number of advantages for internet access. PSTN stands for public switched telephone network. This is the same network you use every day for making telephone calls. The most fundamental one is that it wasn't originally designed to carry communications between computers.
Ordinary telephones convert a persons speech into what is termed an analogue signal.
Similarly on a telephone line the electrical varies in accordance with the same vibrations. To convert from the digital format of a computer to the analogue format required on a normal telephone line requires a special piece of equipment called a modem. The other main drawback of the PSTN is speed. Basically the PSTN has a kind of speed limit imposed on it by the technology used in telephone exchanges. This is adequate for a single computer, and might even support a small number of computers on a LAN, but it becomes extremely limiting if the number of computers in a school grows over a number of years.