If you were president of a club but chose not to continue, your college applications can indicate that you served as president during whatever time frame you did. Period. You need not give a reason why you stepped down. Admission officials are barely going to notice. They will be more focused on what you ARE doing than on what you are no longer doing. They will note that you did hold a leadership role and, if they wonder fleetingly why you don’t anymore, it will not keep them up at night (or impact your admission verdicts). Admission folks realize that students change their paths for many reasons.
However, if you so choose, you can use the “Additional Information” section of your applications to explain why you gave up a presidency. You may decide that it’s important to provide these details, especially if you feel that your reasons say something about you and your values (e.g., “Our faculty advisor encouraged cheating in competitions” or “the Irish club wouldn’t allow any English students to join”). Of course, you want to avoid an explanation that puts you in a negative light (e.g., “Oh, geez! It was way too much work”) or one that seems petty (“My best friend dropped out of the club so it wasn’t fun anymore.”)
If, however, your early high school years were full of activities and leadership positions but, by 12th grade, you appear to be doing very little outside of class, then this may raise a flag for admission committees. But bailing on the top spot in one club won’t make a ripple.