This here's the kind of sentence that's always wagging its index finger in your face. An imperative sentence issues a command or makes a request. It ends with a period or an exclamation point, depending on how forcefully you want to communicate your ideas.
Think of it this way: some authority figure has probably told you at one point or another that it was imperative that you do something. "It's imperative that you do well in Algebra class" is just a fancy way of them saying that "You must do well in Algebra class." It's also an imperative sentence.