yes it is
however I got 87 and I wasn't allowed to do it
simply because I got 50% in the trial sc
Sorry, but if you really wanted to do it, you could have done it. Schools are
not allowed to stop students from studying subjects, they can only RECOMMEND that students not do them.
At Marian Catholic College (my school,
Marian College Online) the Coordinators recommend students do a certain level of English, ie if someone in ENG6 chooses Advanced, it is recommended that you don't do it. A person in this situation would have to sign a contract that effectively says, "I accept full responsibility for any poor marks I might achieve in English (Advanced) that have an adverse impact on my ATAR".
Edit: By the way, I got 88 in the English-literacy test. I was told by the Assistant English Coordinator, Mrs Ryan (admittedly this was before the results came out, but still), "You should be doing Advanced. Do Advanced, if you can't do Advanced, then who can, James?"
By the way, OP, the School Certificate tests are a very bad indicator of your English ability - it's easy to be stressed on the day and hence do poorly in the test. Also, 84 is a GREAT mark... don't let anyone tell you otherwise. If you don't believe me, read the Band Descriptors (these tell you what a Band 1 - Band 6 student can do):
http://www.boardofstudies.nsw.edu.au/ebos/static/BDSC_2009_10.html