Based on my limited knowledge of this, and having been around the process for the IR in the USA, I am with Englishal on this.
The need for a Part 141 school to get an I-20 to get the Visa is particularly nasty because there is no indication on the TSA website whether a school is 141. I blew away a few hundred $ on aborted applications before discovering this.
If anybody wants a detailed checklist for the TSA+Visa process, please email me.
The M1 visa is valid for the study period as specified by the school. In my case (2 weeks) they put down 4 weeks just in case. The Visa also gives the name of the school so you are well stuffed if you can't train there for some reason. Someone here from the USA disputed this; I PMd him for details of how to get the school changed but he never replied.
The Visa process involves a day spent in London, preceeded by about 2 months to get all the paperwork "ducks in line". The TSA stuff can be done entirely via their website, but you have to get fingerprinted, which in the UK can be done in Farnborough, and I gather one other place in Norwich. That's another day trip for most people.