I did debating, mock trial, and mooting. Don't worry much about work experience! I did two during the summer before I applied, but ended up not talking about them at all in my application because there wasn't enough space in my personal statement. Honestly, work experience for law at this stage isn't useful for applications specifically, but just to give you an idea of what being a lawyer is like. Additionally, it is unlikely that you'll be doing anything meaningful, since no law firms will entrust high school students with anything important (at least in the ones I went to)!