We are going through schools, checking out materials and websites etc., and trying to make like a checklist of wants or needs in a school. We have things like - prefer single dorms, wants a certain sport that is not at every school, close to an airport, advanced foreign language. What are some of the criteria on your list that maybe we haven’t thought of yet?
Thanks!
Other possibilities:
-Coed or single-gender?
-size of student body?
-Saturday classes?
-religious affiliation?
-dress code?
-specific EC’s offered?
-arts programs (fine and/or performing)?
-rural, suburban, urban?
seated meals?
student / teacher ratio?
Coolness of dining hall?
If Chapel is required a few times a week?
Beyond size, location and availability of certain academic/athletic/EC offerings, we found that formal meals, Saturday classes, dress code and chapel requirements were all helpful metrics to consider.
In addition to the above,
I had * endowment size * in the school spreadsheet. Just to compare financial health of different schools.
Also for admission chances - * Acceptance Rates, SSAT average, SAT average (graduates), ACT average (if available.) *
Completely forgot about the factors listed by @payn4ward! Yes to those! For AppleKid, %boarding eliminated many, many schools from the initial consideration, as well as ssat and sat averages if they were lower than AppleKid’s local choices. And after M10, endowment size became a more important factor for DH, although the final choice was out of his and my hands 
Languages offered
Arts programs offered/required
Academic requirements stringent/flexible (i.e., Does a student have freedom to take more courses in her area of interest? Or must all students complete the same courses?)
Average class size/distribution–are there small classes of 3 students in very advanced classes?
The boarding/day question is one I fear leads many families to exclude fine schools. For me, that is a subjective, rather than an objective measurement. I know where we would fall on the scale, after years of being boarding school parents. I would not assume that a higher boarding percentage is automatically better. It depends upon the school’s leadership.
Likewise, dress code is one of those things which is off-putting at a distance. Boarding school closets are not large. In the end, you wear the clothes you possess. Whether it’s jeans & sweats or jacket and tie isn’t a huge difference when you’re wearing the same small selection of clothes.
Here are categories which can make a difference. There is no “best” model, though–it’s what you prefer:
Student housing: By grade? Or are all grades housed together? Are freshmen housed in freshman dorms?
Advisors: Assigned? Or are students able to request advisors? Are advisors assigned by dorm? Can students change advisors?
Discipline: Are serious charges decided by the administration? A student/teacher panel? One hint is that it can be the SDC (student disciplinary committee) or the DC (disciplinary committee). Here’s a description of how one school’s committee worked 20 years ago: http://www.isacs.org/uploads/file/Monographs/School%20Climate/Discipline%20Committee.pdf.
I think one of the more important questions to know is if the school is a one strike or two strike school and how that meshes with your own personal philosophy. (I’m on Team Two Strike)
One thing that mattered to me (but I didn’t check before applying) was the health service’s hours and policies about taking medications.