One of the things that it is too late to fix at college application time is having a good high school and community activity track record. Colleges like to see students who have a track record of involvement in their high school and in their community. Involvement outside the high school classroom is an indication that a student is well-rounded in that the student:
Finding time for summer employment and school year activities plus volunteer hours can be a challenge for high school students who are trying to get good grades and have a social life.
Students involved in a time intensive activity like football or band need fewer school activities. The goal for students is to be able to show involvement, volunteerism, employment while maintaining good grades.