There are many options available to you, ranging from the trivial (put a router on your classroom LAN and just connect it directly to the Internet) to the complex (use VPN hardware or software to create a secure, authenticated network between your classroom LAN and remote sites). If your needs are fairly simple, and your budget is constrained, you might want to consider using an open-source point-to-point solution like openssh. The ssh protocol can be configured for port forwarding, allowing most network services to be run through it, and provides a high level of security. Free software is available that runs directly on most platforms (Windows, Linux, Mac), and is easy to configure. Because it is TCP-based, it works quite transparently with most network devices and in the presence of NAT. For a small secure WAN in an educational context, I think you will find it to be a good solution.
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