You are confusing mathematical maturity and level of knowledge.
Those are not the same, and it is with respect to maturity that you already are on a university level, from what i've seen.
The professor (AND your regular teacher at school) can easily help you fill in the relevant bits of knowledge, and you'll master them easily enough.
Hopefully, there ARE weekend classes in higher maths.
In a somewhat longer perspective, your school might be willing to give you, say, a few hours off to attend classes at the university, if they are told from a professional there that you are mature for that level of learning (based on what he has learned about you).
As I mentioned to you, your first step is to have a meeting with your teacher, and see if he is willing to help you contact the university in a constructive manner.
He is the person around you best able to gauge your abilities at the moment, and give a convincing argument to the university why they should be interested in helping you.