New York, New York
I agree 100% with the previous suggestions. Anything like debate, mock trial, or Model UN are great! If your school doesn't offer these activities or you want to explore other options then seek alternative means to practice writing and speaking in public. Go out for your school play, start a public speaking club, run for school council, or volunteer at your local legal aid. Not only do law schools want to see that you participated and practiced these skills but they also want to see examples of leadership. It is not enough to pay dues/show up for meetings - ANYONE can do that. You must participate! These schools want to know that you will be invested if they choose to accept YOU.
Furthermore, BRAG about how these activities have helped you grow as a person! Don't just list your activity as "School Play - Actor." In your essay explain how these experiences have taken you from "ok" to "great," "confident," or "excited" to speak in public. Explain how you have engaged and improved and how you plan to continue to grow as a professional.
Side note: A club is usually easy to start in high school or college. File a form with the school and ask a teacher to "sponsor" the club. Have the club meet twice a month and prepare an agenda - look at a Toastmasters/other public speaking club agenda online for ideas. This is a low cost club that wouldn't require dues or have expenses unless you wanted to bring in snacks or something. Even if you have 3 members and you meet for 1 hour, twice a month, on your application to college/law school you can boast about (1) improving your speaking skills and (2) taking initiative when an opportunity did not already exist.