Oh, I was from the school.
My view is that the level of Mandarin amongst the students is the result of teachers who were proficient in Mandarin but who had absolutely no idea how to teach it well and there was at least one teacher who was just plain lousy/lazy.
It is not because the students naturally disliked Mandarin.
We genuinely wanted to learn it. The teaching was below-par.
Most of the time, all the teachers could muster was to rely on ting-sair or mer-sair.
Also, unfortunately in a school with a reputation for wealth amongst the families of many of its students, at least one Chinese teacher began to discriminate against those from poor family backgrounds (for example, me).
I began to speak Mandarin some time after worklife, when I mixed with Mandarin speaking friends, watched Mandarin movies and Mandarin TV programmes, listened to Mandarin songs and that is the true immersion that leads to natural learning. You pick it up naturally without silly rote-memory methods that kill the interest in the language.