Determine if you want a single-sex or co-educational environment. Since prep schools of both types exist, you can decide if you'd like your child educated around members of the opposite sex...or not. At first stage prep schools (for younger children), a co-educational environment can have little to no impact on your child's education, while a single-sex second stage prep schools are typically thought to offer a better educational environment.
Consider the location. If you want to be able to visit your child on the weekends, then you'll need to select a prep school that isn't too far from your home. You'll also want to determine if a rural or urban setting is better for your child's educational development.
Consider the prep school's boarding requirements. Some schools allow day students to attend classes and go home to their parents at night. If you don't want your child to board overnight, then you'll need to compare only those that offer day pupil programs. If you're selecting a boarding school, you need to compare which prep schools require full-time, weekly or occasional overnight boarding.
Compare accommodations offered to children who board at the prep school. See how they're boarded by age and gender and whether or not they're boarding solo or with roommates. Compare nutritional and activity environments that are provided to ensure all your child's needs are met.
Check the school's inspection scores. Every prep school is inspected by the Independent Schools Inspectorate and the OFSTED. They'll determine the overall impact of the boarding environment on your child's stability, education and development. A quality prep school will have passed with at least a "satisfactory" rating.
Compare pricing on a monthly and annual basis. While cost shouldn't be your only comparison, you certainly need to choose a school that will fit into your family budget...and remember that the schools with the highest prices don't necessarily offer the best education.