Stateful inspection. Unsolicited inbound connections are blocked by default... in order to get around that, an outbound request has to establish an "assured" link first. The game uses UDP for the bulk of gameplay data, which doesn't <normally> get tracked through security layers for the purpose of assured connections. So even though you have established an outbound connection to a player, when they go to send data back to you the connection is denied because security did not track it as an established outbound connection... considers it an unsolicited inbound connection. This is more critical for the PvP stuff because it is so much more dependent on the p2p player direct transfers to keep things in synch. The PvE stuff can tolerate more hiccups with player direct shares via P2P because there is less impact if it has to fall back to the slower refresh cycles of the dedicated cloud and Bungie server connections. Pretty much the most direct way to get around it would be to tunnel to an external proxy device (ie: a VPN). Otherwise they would have to spend time building a dedicated path through their security for you to use... as well as a ton of them for everyone on campus that could run into similar issues with other services. Unfortunately, their are issues with using a VPN with this game... in some cases it would have to be done through a dedicated router based solution (like the WTFast feature of an Asus RT-AC86U) to avoid complications with the game's own security.

Maybe your school thinks you should be studying instead of playing VJO games...