You also have to keep in mind that you're learning more in math class than just math.
While you're learning trigonometry, chemistry, and literature, you're also gaining skills and knowledge in problem-solving, abstraction, communication, citizenship, time management, work ethic, and God-knows-what-else.
And Aquarius is right: you had ought to listen to your dad. If you've already got everything down pat, then heck--move ahead to the uncovered material. It'll help to make everything more coherent when your teacher covers it later. I myself also like making tables and charts with information from the text books and such; the act of reading everything, organizing it, and writing it down helps to get the information to "stick" better, plus you end up with convenient references that you can understand effortlessly.
Also, sit down with your dad and let him know that you're feeling antagonized by him; parents often don't realize just what kind of effect they're having on their kids when they try to give them a push in the right direction. My parents tried to help me along in a similar fashion, but it (combined with other factors) only made me fall into depression from too much stress.
