Senior year absolutely counts! Let's say that you put in your applications for December. Your mid-year grades comes out generally in February. If the admissions office has made a decision regarding your acceptance, the senior year grades to that point, will validate their decision. If they have not made a decision, the mid-year grades will play a large role in the final selection process. At the close of the school year, a final transcript will be sent to the college which you have selected to attend. If your grades are poor, they can pull your acceptance or, put you on academic probation even before they start school. So, yes they matter - they matter a lot!