The tea school that I follow is the Ueda Soko tea school, which has its headquarters in Hiroshima. It's a smaller tea school, formed by the samurai warrior Ueda Soko (1563-1650), who learned chanoyu from Sen no Rikyu and Furata Oribe. In terms of tea schools, even though it is smaller and lesser known than say Urasenke or Omotesenke, it is however a higher class tea school, that is, the samurai class were regarded as the highest class, followed by the peasant/famers, artisans, and then lastly merchant classes, which is where Urasenke and Omotesenke were formed (or so I've been told). Merchants were ostracized as "parasites" who profited from the labor of the more productive peasant and artisan classes. Not only did merchants live in a separate section of each city, but the higher classes were forbidden to mix with them except on business.
There are quite a few differences between the schools, which of course have evolved over the years, depending on the master at the time changing to suit their requirements, as it is an ever-evolving art.
Some interesting facts: only Uransenke usually call the Way of Tea 'CHADO', whereas all other tea schools say 'SADO'. When preparing koicha, Omotesenke are the only school that uses an usucha chasen, instead of the usual koicha chasen, which is used by all the other schools, which have thicker and therefore a lesser number of tines than the usucha chasen. IMHO, ALWAYS use a koicha chasen for making koicha! They may cost more, but they produce the desired result. Furthermore, when making koicha, (in our school anyway), if you're looking down into the chawan, you're whisking from 1 o'clock to 5 o'clock (For one person, 3 1/3 chashaku scoops and whisking 50 times).
I hope that sheds a little more light on koicha, tea schools, and how it evoled. Happy to answer any other questions people may have.