I was fortunate enough to be able to go to a 2 day M School last March. It really was as good as the videos and reports say!
Day two of my school, when we were doing full track lapping sessions in the M3 and M5, was non-stop medium to heavy rain. I was truly amazed just how hard the cars can be driven in conditions like that. That experience gives me a huge amount of confidence in my M3.