Visa question.

If I was scouted [for a talent agency] in Shibuya while on a student visa, is it possible for me to work, or would I need to apply for a different type of visa?

You will need to apply for permission to work at the the Immigration office. You don't need a different visa necessarily, but by law you do need this endorsement from the Immigration office- head to Shinagawa and fill out the forms.

With a student visa, you need to apply for:

"Permission to engage in an activity other than that permitted by the status of residence previously granted"

I know long title right?

Which will allow you to work part-time if approved.

Not only do you need to apply for approval, but you need additional documents from the school, showing you have good attendance and grades. Generally speaking if you are within your first 3 months on your student visa you can't apply for activities until 3 months after.

Your school could help you with any additional documents and the immigration office should be able to tell you if any other supporting documents is needed.