This is correct as far as i knowAs far as I knew you can drop any subject besides English up until the day before the HSC exam without any penalty? Of course you have to have enough units to drop though.
from http://ace.bos.nsw.edu.au/ace-8068Students may withdraw from an HSC course without penalty at any time before the HSC examination. Prior to the due date for submission of school assessments to the Board of Studies this is done by the principal via Schools Online. Later withdrawals must be notified to the Manager, Records, Credentialling and Reporting Branch at the Board.
I've talked to my teacher about this, concerning MX2, apparently there is a final date but it's quite late, towards the end of term 3 I believe.