Changing VPN Listening Ports Windows Server 2012

Changing VPN Listening Ports Windows Server 2012

We have a Windows Server 2012 vpn setup at the office, but we've ran into
a problem. We often work at a school and we can't connect to our vpn from
there. Anywhere else we can connect perfectly fine.
It's the built in VPN server.
I have looked online for how to change the listening ports and most people
just say use openVPN instead...
How do I go about changing the ports Windows Server listens for?