The answer, for FortiNet, is almost certainly "no". Apple's built-in VPN client from this era supports PPTP and L2TP and maybe one other things, but as mentioned in the intervening decade, ciphers have been updated.
FortiNet (and Cisco AnyConnect, as another example) is another kind of VPN sometimes referred to as SSLVPNs that talk between a proprietary client and the server via, you guessed it, SSL. (In particular, these often use port 443 to avoid being caught by firewalls that disallow "VPNs". Most of the time, there are not third party alternatives to these kinds of VPNs, and if they were, they wouldn't work on Mac OS X 10.5.
You'll probably need to use a more modern computer for your schoolwork.
Just idly, (this is basically unrelated to the original question here) PPTP is pretty easy to set up as a client on Macs, but it did get dropped out several years ago because, well, it's been cracked for a very long time, so using a PPTP VPN for anything is going to be more about "not opening other ports" than about "actually encrypting traffic."