First step is to eliminate any rivals.
Challenge one of the popular kid's best friend to a duel, after eliminating him you must sever his head and taunt the popular kid with it. In his rage the popular kid will charge at you, eliminate him with your mafia squad. In the confusion and chaos that follows numerous warlords will challenge you, take the strongest warrior as you best friend, grab a few other allies, and even a third wheel if you want. As your power grows, you will be attacked by beings known as "teachers".
Slay them as they come, then a friendly constable will arrest you. I'll send Mr Fujita to you, and you'll both run to the Bahamas to join FRIDAYCorp. Queen Rebbeca of Black will await you there where you will undertake various missions to fuffil the goal of returning the Gem of FUN to the temple of Cereal. You will then be given the choice of taking any seat you want.
Source: Personal Experience.