I find this story refreshing actually...(whether private schools are good/bad is a different discussion)
I would not get too excited, BazFitz can correct me but I would speculate that there is a feeder primary school attached to this secondary school and it has high fees. Otherwise there is 30 students or one class in each year which I think is unlikely.
There are many public schools where demand exceeds available places and they appear to set a variety of criteria from geographic area to ethnic diversity to family connections. Selective entry is not the preserve of private schools.