I have a 6 month contract offer from a school in China. They say they can help with getting an F Visa, which is a business visa, and that Z visa's are more used for 1 year contracts. Is this correct? I was under the impression that all teachers needed a Z visa no matter how long the stay?

Lots of my colleagues are on F Visas. My lodger will be on an F Visa. You can't blame the bosses if they don't want all the hassel of obtaining Z Visa, Expert's Cerificate and so forth; especially when there are so many fly by night young FT's who are likely to scarper after a couple of months.