A melamed may learn most from his talmidim, but I will be willing to conjecture that the Gemarah in Taanis was not referring to whether or not a certain bochur should be admitted to Yeshivah, but rather to hashkafa and ways of looking at an inyan in Torah. What the principal is doing is out and out Loshon Hara, encouraging the same in the boys to whom he is speaking, and shows him to be singularly unprofessional and not competent to do the job for which he was hired.
BTW, I’d feel differently if the bochur is accepted into the school and then starts to act out after that or has some problems. Then, it would be very proper for the mechanchim to speak to the boys’ friends to see if they are aware of why the boy might be having a problem. Of course, they should talk directly to the boy and his family first. But in this case, the boys might know something that is currently going on in the boy’s life, of which the menahel is unaware. But to ask them to speak about some 8th grader who is not even in the school yet… UH UH!