A) what exactly do YOU call it when an adult seeks the advice of a teenager?
B) Who cares HOW well they THINK they know the young person in question? That is completely irrelevant. How do you think the child being asked for his opinion would feel, were he to know someone was talking behind his back to someone who had power over his future, with the potential for it being extremely unhelpful to him? Better yet, how do you think his PARENTS would feel about it?
I would speculate as to your own age, given your position in this issue, but the truth is, a responsible person of any age would see that at best, this is a very poor example of chinuch, and the principal is showing himself to be singularly lacking in good judgment. I would even venture to say that were any child not accepted in a Yeshivah because of something some kid blabbed about him, I would think that the parent would take the school to a Din Torah, were they to find that out (and given how immature kids talk way too much, it would definitely get out at some point).