My son is doing Grade 11 next year, and I completely understand the pressure you're talking about.
We just tell our son that what is important is that you try your best and anything less is just letting him down. If he tries his best and comes out with lower marks than he expects, that is FINE. There is no such thing as a fail, especially when the effort has been put in. He was worried about not getting high enough scores to get him into Uni for the course he wants to do. I told him that the worst case scenario would be him working and perhaps travelling until he could enter Uni as a mature aged student. He can also do some study at TAFE.
There is definitely so much pressure on them to do well.