You now know that there is interest in your school for a MUN Team, but now you must make it happen.
Most schools have procedures for starting new organizations or clubs.
Speak to a member of your student council or possibly the principal or assistant principal. Any of these people could help you with the specifics of starting a new organization at your school. Next, you want to consider the structure of your team. This may depend on how your team will be chartered in your school. If you charter your UN team as a club, then you may want to have elected positions with annual elections, open membership, and an advisor.