Hello,
This month, I am taking thaï courses at Bangkok. I advise my School: Bunditwit Thai language school: http://www.blsthaischool.com/
I think it's the best. It is very close "Sala Daeng" BTS station. Good method, good teachers, good atmosphere.
We are all very pleased with our progress. The school offers sessions a month with all morning or all afternoon.
It is not expensive and it really deserves it!
(I'm french, sorry for my bad english)