Everyone gets depressed, and it sounds like that is the case with you spunky. You didn't say your age, only that you miss school. School can be overwhelming for a lot of people. Homework, extra activities, your peers, and whatever else you have going on can get the best of you. You should talk to your parents about this, and if you can't talk to them go to your guidance counselor, school advisor, or your family doctor. You should talk to an adult you can trust. They may be able to give you some insight into what your are going through, or give you a crisis number for your area, so you can tell someone how you are feeling.
Depression usually doesn't go away on it's own. Good luck to you!!!!