Lauren as a maths tutor I have contacted CCEA regarding grade boundaries. If you are getting high 60's as a raw mark in the test this will be moderated by the examination board when you do the exam. This equates to a mark in the low 80's which is a "C". Its some statistical process they do so don't worry. Keep your head down, again don't worry and go in and enjoy. Worrying never helps. Good luck.
Peter