First off why would you be giving guests and teachers unfiltered internet? This poses both a safeguarding and security risk. Think there might need to be some political work done here.
secondly, you will need to get the company who manages your router to do it. On the switch to make it non-routable just go into the VLAN config and issue the command disable layer 3, you can then enable this for certain VLAN's even narrowing it down to certain protocols.
Check the router port is configured properly as a trunk port and the port on the switch should be an access port. If you set it as untagged this will keep it super simple. Do you have a support company as this is a super simple task and I'm sure that can show you how to set it up. Other things you can look at is switch and router firmware. Even a tiny school should have 4 VLANs (flat,wireless,phone,servers) this obviously reduces if you don't provide all of these services
