I’m a bit thrown off by the question that asks about the “school or summer activity in which you have been most involved.” Most of my activities are year-long and outside of school. I do have some things that fit in the either-or category, but they aren’t what I’ve been “most involved” in. Help?
The activity I wrote about was outside of school and year-long, but I just chose to write about the one that was most important to me and the one I was most involved with. I think they want to know which extracurricular is most important to you, what contributions you have made, why it is significant, etc.
I think you are getting side tracked by the word “summer.” My reading of the question is that they just want to know about an EC that meant a lot to you and where you had significant impact.
I think an activity that took place outside school would be fine, and might even stand out from all the typical in-school activities.
Best of luck.
Perfect! Thanks 
@randomstudent789 @Dancingmom518