Since there is no detailed plan, I will come up with some.... pick one and decide between urselves
Dorm War (Fortress)
Each dorm pick 3 ppl representatives for them and label each one of them with either "Armor", "Gadgets", and "Core". Each dorm's "armor" faces each other first, then if armor is defeated, armor faces that team's "Gadgets", "Core" will be the last defense line. Once "Core" lost, the dorm loses. This will have a round system, meaning it will be "Armor vs Armor" 100% 1st round.
*Special rules and handicap can be made for "Gadgets" and "Core" to balance the teams out.
Dorm War (Crossfire)
All the people will need to sign up for joining, then we will count how many people we have for each dorm. Well, it's obvious that we have less Ra than Slifers, so by crossfire it will mean something like this: if Ra have 3 ppl and Slifer has 30, each slifer will only have 1 life while each Ra will have 10 lives.
*People in the same dorm can share their life if both players agreed.
Dorm War (Life Point System)
Since we have way more slifers than the rest, in our to balance out the number, reds will have a civil war and the survived ones will face the other dorms. Though the charm point of this choice lays here, each players left out in the real war (not the red civil war) will have 4 chances. What I mean is this: If a Red vs a Ra and win, the Ra will have only 3 chances left before completely defeated. Next match the Ra will have 6000 LP only while the Red has 8000 still. (and so on 8000 >6000 >4000 >2000 >OUT)
Dorm War (Handicap)
Reversed version of the Life Point system one. Reds(and/or blue) picks out the equal number of people with other dorms. Then each have 4 chances, but in this one the one who win will have 6000 LP (each victory -2000LP) and the loser will have 10000LP (each loss +2000LP).