My school already has mock trial and debate team
I’d just join one or both of them and show leadership and get elected to an officer position.
If you are dead set on a new club model UN is a good option though debate and trial are much for law based
If you're opposed to joining one of the existing clubs and trying to attain some leadership in those, a Model UN chapter would be a good option. However, I would still join both of those clubs no matter what you do otherwise. Colleges know what's available at given high schools and if you write an essay saying that you're really passionate about law and want to study it someday, but they see that you didn't join the mock trial or debate team and instead started a Model UN club, they're going to interpret that as a choice geared specifically for college applications. (That's not a good thing.) If you do all of them, you don't run that risk.
Outside of starting clubs though, I would get more creative with this. Try to involve yourself in political work — whether that's activism, interning or canvassing for a political campaign, or engaging with the judicial system in some other way. Courthouses hire high school students for internships and even regular jobs. Think outside the box with this.
If you chose not to opt for a common, more typical, law/ government related club, you can always reach out to local law firms ( vary in law practiced, or concentrate on one) and do a sort of mentorship club for all students interested.
To keep this community safe and supportive:
I would greatly emphasize founding a Youth In/And Government program at your school. I have nothing but fond memories, and it has both legislative and judicial activities.