most of the freeway bus services from doncaster park n ride to the city (melbourne central/myers) are about 22-30 min according to their timetables, even in peak hour.
This is just as quick as the train from neighbouring box hill.
Good student results is better for the reputation of the private school and future income (fees).
They don't want deadwood dragging down the average. They are a business.
They would have ways of weeding them out.