Charter bus options for school field trip?

I am trying to arrange transportation for a group of 50 (charter-school) third graders to Stone Barns in Tarrytown. Would prefer not to use the city buses (we've had problems with promptness before) or one of the mobbed-up private operators (Jofaz, etc). Thought perhaps a PTA parent out there might have a suggestion for an economical, prompt school bus or coach bus operator, etc... Thanks...

I've had good luck with J & R Tours

http://www.buses.com/