I understand that but we have never had any issue with https and as far as I am aware we have never purchased an SSL cert either.
Just access them the school doesn't host anything like that, Is that what the certificate is for then to access hosted sites ?? I suppose I would need one for when I have Smoothwall installed so teachers can access via the VPN, but I am led to believe that Smoothwall will create its own cert for the VPN.
You only need to buy one if your hosting the kit.
Smoothwall will have a self assigned certificate, you can install this on the machines so it trusts the smoothwall box.
You should go for direct access rather than smoothwall. The teachers just turn on the laptop connect to WIFI and the VPN connection is Automatically established.
scenario for you:
If you have an intranet that you happily use then you decided to open it up so you can access the system externally (from home for instance). If you have a https: page for logon then a "self certified" certificate will flag up big warnings to the PCs at home (since they arent on your domain so they dont understand that your server is "trusted"). A purchased SSL certificate will prevent this. Of course you can tell your teachers to ignore this warning - however this is a *bad* thing to get the teachers into the habit of doing...