You may want to seek a program with somewhere in the range of 400-650 program hours, including "hands-on" time; at least one teacher certified by a nationally-recognized body; and good hands-on / hands-off, student / instructor, and student / dog (practice groomings per day) ratios.
Look for schools that provide clear breakdowns of their training programs, which include areas such as tool selection and usage, health and safety procedures, clipping techniques, and styling profiles for different breeds, among others.