Usually it is a headmaster's reference, so whilst the class teacher may have input it would be the head who would be submitting the report. Usually the school puts most weight on exam and interview so the references usually just tip the balance for borderline candidiates.
"Antisocial" would ring alarm bells. "loner" wouldn't. But if a child had passed the entrance exams and the interview then I would expect the school to talk to current head if the reference was particularly negative.
It tends to be a private school thing - locally at least half the state schools fail to provide references which isn't seen to be a problem. If switching schools midyear then the new school will want to ensure that you're not leaving behind a huge fees bill. I had to supply 2 years worth of school reports too.