There's a common misconception that once you've made it to your senior year -- and especially once you've been accepted by colleges -- that senior grades don't matter. But colleges do request and review senior grades, especially from the first half of the school year. Be forewarned, though, that if you let your grades slip anytime during the year, colleges have the right to ask you to document and explain why your current grades have dropped below previous years.
Continue to take pride in your grades and continue to attempt your best. Some studies report that high school seniors who lost interest in high school because of a “senior slump” also had problems in college because they had lost interest and devalued the importance of education. And you don't ever want to face the worst-case scenario: not graduating high school because you are short one credit because you failed some elective course.