mjhjmtu

I want to set up multiple Raspberry Pis to be accessible via VNC. For this I think guacamole is a good option as it doesn't require to install a client which is really what I want.
The thing is that the pis are behind regular private routers without the possibility to forward ports.

There's a regular server that's freely reachable from the internet that I could use to tunnel the traffic of the pis. But because of the applications running on the pis it's important that they keep their own IP-adresses and so I can't use VPN to the remote server.
My current idea is to do a reverse SSH-tunnel from the pi to the server and then configure guacamole on the server as if the vnc-port was on the server itself. This is described here.

The question I have now is:
Will this idea work for my explained plan? If yes: is there any possibility of only one port or something?
I'd like students to just change credentials on the raspberry, plug it in and use it out-of-the-box. But if I do this reverse tunneling stuff I have no idea how to configure the port the pi should use for the reverse tunneling.

Edit:
Here's a quote from the linked page:

workpc -- linuxhost || ooooooooo || pi -- homepc

the reverse SSH originates on linuxhost over Internet to the Pi:

linuxhost$ ssh -fN -R 7000:localhost:22 piuser@homeip

Then from my homepc, open a SSH connection to the Pi forwarding a VNC port:

homepc$ ssh -L 5901:localhost:5901 piuser@piip

Finally, the Pi opens a SSH connection to the Linux host, forwarding the VNC >port to my workpc, all through the reverse SSH tunnel:

pi$ ssh -L 5901:workpc:5900 workuser@localhost -p 7000

Then from the homepc, simply opening a VNC viewer gets the work desktop:

homepc$ vncviewer localhost:1
I'd skip the homepc connection and replace it by the guacamole-pc. Rest would be pretty similar, but that linuxhost and raspberry pi should be switched, right?