When we wanted a Montessori-type program for our 23mo daughter, I toured several in Coppell and called several more. I wasn't so concerned with teacher/student ratio but I wanted a place where there was continuity in the staff, a challenging curriculum for the kids, and a good part-time schedule. When I found Carrollton Montessori Dickerson (just east of 35 on the north side of the George), it was a done deal for me -- quiet, well-behaved students, all the usual Montessori activities, good school nutrition guidelines, good schedules available, not out-of-line tuition. In my daughter's room, the lead teacher has 31 years of Montessori experience and 18 years at that one facility, and her two aides have each been there at least six years (ratio of at most 6 babies to 1 caregiver, but I never seem to see 18 kids there). I mean, it's a SCHOOL, not a daycare, even though the kids go down to 18 mos. My daughter's been going there a month now and she LOVES it! They're part of a big Montessori network; visit http://www.montessori.com/index_home.html to access the list of local facilities.
There is another Montessori almost directly a mile south of this one, on the other side of the George. I didn't tour it and I didn't call it, which means they didn't have a part-time program that worked for me, they weren't recommended to me, or they didn't have any Web presence (I at least called every one I found on the Web that posted a workable schedule). It's called Mona Montessori or something like that. Just fyi.
Please feel free to reply privately if you want to chat more about my experiences touring other facilities in the area. Hope this helps! :-)