What are the requirements for visa extension?

I am currently holding a long term visa (category no. 6), and I think I need to study for me to extend my visa in the future, though I don't really know if it means that I need to attend a High school, college or, attending a language school is acceptable for my visa extension.

Additionallly, what do I need to pass, to prove that I studied ini a language school. Like for example, do I need to pass an attendance certificate?

Assuming that you have / want a resident status as student, then you need to study in an institution that is officially allowed to sponsor your visa. That could be a language school, university, senmon gakko…
And yes, you need to attend very regularly. Attendance record will be taken and if you miss class too often you can loose your residence status.
The school will able to tell you the threshold, but essentially you need to plan to go every day so that should you become sick or have some other extremely urgent rain not to go, you can miss a day without risking your visa.

Are you going to renew your long term resident status, or changing to a student one?