Evidence from school projects in the United States shows that one modem per school is not enough: multiple connections to the Internet are needed for significant learning gains.
The options depend on how many simultaneous connections you want to support and the number of hours' use per day. If you are in an area with free or cheap phone calls, a rack of 28.8K modems to connect your network over dial-up phone lines makes economic sense, though the connection will be less reliable than over faster digital networks using ISDN or a leased line.
If you pay for calls at business rates in the day, a leased line to your Internet service provider may be cheaper. For example, a college in Coventry wanting to connect its network for more than seven hours a day, five or more days a week, found it cheaper and more reliable with a 64K (kilostream) permanent link to JANET, the joint academic network, via a local university, rather than multiple dial-up connections to a commercial supplier.
A 64K connection will, in theory, service about eight connections comfortably, although users get a thinner slice of the bandwidth. Since the traffic is usually in bursts, some users will not suffer a particularly noticeable loss of performance unless they are downloading a long file or viewing high-graphics Web pages, which will run slower.
For Pounds 2,000 and a leased line, they bought a permanent connection to the Internet that they could use as much as they liked at a fixed cost. A similar connection from commercial suppliers would cost a couple of thousand more. An ISDN line (64K based on connection time) would cost them Pounds 5,000 upwards for 40 hours per week use.
As more machines are connected, it becomes desirable to increase the bandwidth to avoid congestion. Upgrading ISDN connections to services such as ISDN 30 (multiple ISDN lines) or leasing more powerful lines from BT becomes a very expensive option. But as whole regions get cabled, this may become more cost-effective.