While I believe CAP and JrROTC are excellent programs, and they definitely let you get a "PEEK" into the military (Small peek); it is not "Special Block" that looks any better on your application than most other activities. You don't get more points for CAP/JrROTC/Scouts than being involved in Student Government, Academic Teams/Clubs, Debate, Internships, Culture Clubs, School Newspaper, Sports, and an endless list of others.
Too many people have a misconception that because CAP/JrROTC/Scouts wear military style uniforms and even has some military affiliation, that it somehow gets you "More Points" on your academy application. IT DOESN'T. If you WANT to get into CAP/JrROTC then do it. If it's not of interest to you, or your time is restrained because of other activities you'd rather be doing, then DON'T get involved with CAP/JrROTC.
The key with ANY of these activities, is to grow in them. Expand your TEAM MEMBER Skills; and advance your LEADERSHIP Skills. Drum Major; Captain of the Debate Team; Captain of a Sports Team; Class Secretary/Treasurer/VP/President; Head Cheerleader; etc... ALL LOOK BETTER on your application than just being a MEMBER of CAP/JrROTC. And starting in your Junior Year something like CAP/JrROTC is probably not going to get you into a leadership position.
But if it's something you WANT to do, then you definitely should. For whatever reason. Just don't believe that it somehow is better on your application than any other activity. Again; LEADERSHIP in the activity is more important than the activity itself.
And for what it's worth, I help a lot of kids apply to traditional universities besides just the academy. And all of the universities, including the Ivy's and prestigious universities, love extracurricular activities. And the ones looked at most impressively are:
Academic Teams and Clubs
The Arts: Music, Drama, Visual
Volunteer Work/Community Service
Part Time Job
Does this list look SIMILAR??? It should. The academy looks at these same types of activities. But again, with the academy, you could add 100 other activities to the list, and what is important to them, is that you built team building and leadership experience in the activities.
So join CAP/JrROTC if you want to. But don't believe that it's a SPECIAL ACTIVITY that's going to get you more or better favor on your academy application. It doesn't pull any more weight than any other activity. It's the leadership and team building you do in it that matters.