Piano lessons are going to take up lots of time (if you're not doing music for HSC and just for personal purposes, I think it would be more suitable if you maybe sacrifice 1 year only because it would save a lot of time and stress). I advise to focus on your HSC for this period if you are aiming for a good ATAR, but if you love Piano then go for it!
From my personal opinion, I find 14 units a real time killer, that extra 4 units could be spent well on those 10. I would only do 14 units if I wanted to take the safe way out (If you're not very confident also), but with good study habits and time efficiency you could do a lot better with 10-11 units. Workload and stress wise, 14 units is a lot to handle also.
I would say try it at least give it a go and if it doesn't work out, drop whatever subject is the weakest and you do not feel comfortable or confident with. I am not sure if you should drop Ancient or 4U, or keep both, this depends on your marks and how well you are at these subjects. But I think Ancient is a very good subject and you could easily score marks, it's a very straightforward subject as I did it for my HSC.
Lastly, Extra curricular activity is always a good compliment and will benefit you a lot for University You would also fit in much well with the societies and clubs also!
Yeah that's the issue, cause it's not one of my subjects for the HSC, and it takes up 5-10 hours a week, but I thought maybe having piano lessons it would be a break, so I have something to go to that isn't school related, but I'm not sure. I think 14 is an extremely safe option, so I think 11-12 would be better and less stress on me, although I'm just nervous which I will flunk out it, do you reckon I should just wait for my results in the year 11 yearly exams to make a decision?