Sore lower back for months

For a very long time, probably since November-ish last year, maybe even before, I've had a super sore back problem. I'd sit down at school and if get back up and my back would hurt a lot. I'd dread standing up because that meant having to deal with the huge amount of soreness. Sometimes as I'm sitting it kind of radiates up my back. It's like using a sore muscle, but only after getting up after sitting for awhile. It's been really annoying me. (Fyi-I am an early teen and don't have any certain doctors or anything.) Thanks!