The school administrators block off facebook, myspace, tumblr, ect because they are afraid that it will ruin the education that teachers are willing to give. These sites will distract you from what you do at school and make it harder to learn. You won't be focused, you won't turn in homework, you'll fail tests. You'll fail your classes and might not get through school. School offers a variety of classes to learn for your future. Sure algebra may not seem important to you but it can be a requirement to graduate high school or you may need it to get into college. I know I am probably sounding like a teacher but this is just what I know. And what i'm assuming.
Like Rob said, you need to know what your school has taught you so you can later on do something, for a job, that will earn you money. Say you need to write out an excel chart for someone or your job. If you don't do it, you don't get your check. Last year, I learned the basics to computers and learned how to use excel. So later on in the future, if my job requires an excel chart on oh lets just say, the population of gang members in the past few years. A cop might need to do this. I'm just saying though. You might think that school is stupid now, but trust me, later on in life, you'll regret not taking it seriously.