Thanks Alexander - we crossed posts so I hadn't seen it was a performing arts school.
Agree teaching quality is the absolute key to students' success. Very bad luck indeed to be landed with a poor teacher. Even small group sizes can't make up for poor teaching, nor can much actually.
Agree about luck generally playing a part. It does require luck for all circumstances to be ok at the time of exams and in the run up, whether that's a relatively quiet house, lack of domestic issues/ arguments, no illness including no hayfever/ period pains etc. A lot depends on luck.