Students who are planning on taking it easy during their senior year need to compare their college goals to that plan. The higher their ambitions with regard to middle and top tier colleges, the less likely a light senior year is a good idea.
College admission officers at competitive schools are not impressed by senior year schedules that are less than full time...unless there is some other academic activity filling the gap where the missing classes should be. Some students take a less than full time load during senior year to take college classes, for example. Others are engaged in other advanced activities.
In any event, a student who takes 2-3 classes during senior year with no other academic activity to fill in the gaps should not be surprised if college acceptances are disappointing. Keep in mind that college admission officers are looking for students ready for college. Those who decide to take an academic vacation during the senior year of high school have been shown to do less well during their freshman year in college.