Best visa for a 90 day stay?

Hi there,
I'm a 20 year old British student who will be travelling to China to take up an unpaid, volunteering position in a school in Suzhou. My stay will be up to 90 days, so I'm just wondering which type I should apply for?

I am able to provide all the necessary documents, I just want to have the greatest likelihood of getting a 90 day one, as it seems to be becoming more and more difficult. Today, I went to the CVASC to apply for an M typein person with all the right docs, including an invitation letter with the chop and all the correct info, but they still said I needed a Duly Authorized Unit letter. Should I instead apply for an F type? I just don't get what they want exactly, because the official website isn't updated as changes are happening, and the rules seem case-by-case so it's impossible be sure.

I still have time (travelling on or around June 10th), but I'm starting to worry because I don't know what they want! So if anyone has any advice/knowledge or has applied for a 90 day M/F type from the UK recently, I'd appreciate any help! Furthermore, I've got friends in China who can supply the right documents if necessary, so if there's any other way I can stay for 90 days I'd like to hear it!

Also, I have a part-time job working at a cinema, should I give this as my occupation rather than student? The guy at CVASC seemed a bit off about me being a student.

One last thing, I visited an agency later today and the woman there said there is a 'short work' type which isn't openly talked about but can be granted for 60 day stays...anyone know about this?

I just want to understand what to do! The rules are changing and I can't keep up!

Thank you,
Joe