I completely agree with the club angle.
Have you tried looking through commitees for your local city council? Have you checked with your school to see if they have leadership programs affiliated with them? They may have a leadership certificate program. Or, you could volunteer as a girl/boy scout leader[Like critterfixer said]. If you spent some time in scouts, that would be considered leadership training as well.
I was in scouts, many moons ago. Both boy and girl (long story). I was a girl scout leader ( daisy, brownie, and junior). I went through BSLT (led cub and boy scouts), law enforcement explorers (yes, I can show you where to park), outdoor adventure explorers (explorer vice president of my boy scout council), and I was on an ad hoc committee for my city.
I also went on Illinoisleadership trips (which help to build confidence in a team, and motivation strategies).
In the workplace, you can show that you work well in a team, and know what needs to be done. Good work is recognized, and you move up to those leadership positions. You could put in your employment descriptions where you started out and where you ended up. It would show that you grew and have a concept of a group dynamic. Another way would be to show what responsibilities were given to you over time. I know the boxes are small, but you could fit a little of that in.
I know I didn't put half of this information into last years VMCAS.
The opportunities for leadership are out there.
Good Luck
😀