This seems like a no-brainer, but important enough to emphasize here. If at all possible, partially setting up the venue before Friday comes. Failing that, start setting up early on on Friday. Including tech test time, it can take several hours to make sure the venue is fully up and running.
Don't overestimate how many people can "comfortably" jamming in that space. In a typical classroom setup, if the room was originally intended for 60 students in full capacity, then for game jam purposes, it's better to cut that number in half, say 30. The jammers will be there for not only 2~3 hours, but 48 hours. Participants need a lot more space to put their belongings, equipments, standing up relaxing... etc.
Make sure you check with the venue management about the power capacity before setting up! A game development-capable laptop can use up to 2 Amps of current. If the venue has only 80 Amps of capacity in total, it means the safe upperbound of the number of jammers is 40, give or take.
2 Amps per laptop is a bit of over-estimation, but you need some margin for people who bring desktops, monitors, additional electronics and the organizers' machines. Also, VR-Ready desktops are very power hungry.
When laying down extension cords, tapes and/or cable ties will come in handy. Keep cables away from the main walkways, or at least keep them stick to the ground using tapes.
More importantly, averaging out extension cords connections! A lot of times the power capacity for a room will be divided into different circuitries. DO NOT connect all extensions from a single source unless you like the smell of burning wires.
If the venue you are using is big and you are expecting a lot of jammers, asking your venue management contact for a power circuitry layout map for planning ahead is recommended.
Checking network requirements and access for coproate networks. Do you have to sign in? Do you need people's email addresses? Is there any restrictions on what you can access on the network? Are there any addtional rules the jammers need to follow?
For venue that has auditorium space, always remember to test the projectors, also make sure you have all the video adaptors you gonna need for jammer presentation purposes.
If you plan to do video streams from your site, please see: Streaming your jam on Mixer