I was involved in a community-oriented but school run leadership group freshman and sophomore year; however, as a junior I have not really participated in it at all. The catch is, I've been telling my parents that i have been involved all year to get them off my back. This will inevitably lead to me writing up my application to college and having to write that I did this EC junior year.
What are the usual policies/repercussions for a one-year stretcher lie? Also, what are the chances I get caught? I'm applying to mostly top 25 universities, what's the play here?
It's not worth it to lie about something like this on your college application. It's unlikely that you would get caught, but if schools did find out it would make them think twice about admitting you. The difference between two years of participation in one extracurricular activity versus three years is a minor distinction to make on a college application. The difference between a prospective applicant who is honest and a prospective applicant who is willing to lie to get ahead, on the other hand, is much more significant. This is especially true at top colleges - these schools are looking for academic excellence and strength of character.
If you don't think you can tell your parents the truth without severe consequences, you should fill out your college applications independently so that you can at least be honest on the official record. If your parents insist on reviewing your applications, you can always stretch the truth for their benefit and then make edits that accurately reflect your extracurricular involvement after everything has been approved.