If you
must repeat a year, I highly suggest making it Year 12 and not Year 11. I can't think of any real point repeating Year 11, but doing Year 12 a second time would mean you've been through the Year 12 experience already and (hopefully) know exactly what your plan of action is for your second time.
Possibility 1: Underperform this year in Year 11 --> Catch up in time for Year 12 (you have the whole of 2017 and the holidays to do this) and reach your goal.
Possibility 2: Catch up in time this year (especially since you are not doing a Unit 3/4 subject, your scores this year don't directly contribute to your ATAR at all) --> Reach your goal in Year 12.
Possibility 3: Underperform next year in Year 12 --> Learn, repeat and hopefully manage yourself well enough to reach your goal.
Although your situation is different, there are certainly people who wasted their time in Year 11, but got themselves together in Year 12 enough to do
very well. It's never too late. Also, teachers can be pessimistic about stuff like this. Just prove them wrong if that's the case
Best of luck.