Here are some questions I can think of:
“can you tell me why you think I/my child wouldn’t be a good fit?”
“are there areas I can improve on to increase my chances?”
“are there schools you can suggest that you think would be a better fit?”
If I applied for a job that was a promotion at work and management told me that I wasn’t a good fit or I wasn’t ready for that position, I wouldn’t say “thank you” and get up and leave. I’d ask some questions. (and maybe go cry in the bathroom after 
)