Keep in mind that you are taking harder classes, so this is not unexpected.
Also keep in mind that your study strategy that worked for freshman year may not work for junior/senior/college so you may have to step it up.
Here are ideas:
http://talk.collegeconfidential.com/high-school-life/2026961-what-to-do-when-you-arent-doing-well-in-a-hs-class-p1.html