Continuous headache for a 14 year old girl?

She is 14 years old and has had a headache for about a week. Its one the top left side of her head and it hasn't gone away once. Its all day everyday. I don't think it could be a brain tumor or could it? i don't want to overreact but what could it be? should i take her to the doctor or just see if it will go away? could it be anything serious?