In Year 12 any student that has consistently worked hard right through the year has obtained A+. Any student that has not consistently worked hard right through the year has obtained less than A+. It is simply commonsense.
This can be explained by saying that everything that can be said about sport, you can say about maths and science. That is, if you want to be better at sport you need to practice, if you want to be better at maths and science you need to practice. Practice makes perfect.