[japankyo]

If I understand your question correctly, I think in order to extend all you have to do is show proof that you are enrolled in a language school. F.Y.I. I've read there is a two year limit to staying in Japan as a student.

I received a student visa on my own, just by first enrolling in a language school. Then bringing the certificate of enrollment to the immigration.
Two weeks later I had a year visa.

Hopefully this helps!