Secretly ally yourself with a nearby tribe. Then gang up on the weakest one in the class. Split the winnings. Become unstoppable. Then turn on you buddies during the last battle(Before they do).
Once the rest of the class realizes that you two have allied, then they will most likely sign a mutual defense agreement in an effort to preserve peace. They will never fully trust one another however, and that could be used to your advantage. Start having conversations with one of the weaseliest members of the alliance. Don't discuss the game, just make it look like you are. After a while the other alliance members will suspect that said tribe is talking with you, trying to join your side. Hopefully, if you play your cards right, this will reduce the alliance to paranoid bickering.
But this isn't the time to strike. If you attack one of them when the alliance is weak, all that it will do is strengthen their resolve. You will lose to the rhetoric of "This is what he would want us to do!" and quickly find you and your buddy fighting a war against a more determined foe.
Instead wait for the paranoia to subside. What will happen is a state of unspoken support. They think that their alliance will come to their aid in case of an attack, but they can;t be sure. Meanwhile, your period of peace will give them a chance to let their guard down. That is when you start striking weak targets again.
As for domestic issues, you have to gear your population towards aggression. Think Klingons. Get them drunk, get them rowdy, and get them breeding. Whoever makes it out of childhood should be a drinking, fighting, fucking machine. Make sure to establish an aristocracy in order to govern said rabble and encourage formation of clans and familial careers.
Good luck.