If your teacher can't answer this question, then the biggest favor you can possibly do for yourself is to get informed very well about all those issues before you acquire bad habits. Get the book "What every pianist needs to know about the body" from Thomas Mark (Amazon has it). If I had had this book from the beginning, I would most likely not have had that many injuries (both from piano playing as well as from every day activities).
To answer your specific question. Your head should be balanced over your spine, which should be balanced over your sitting bones, which should be balanced on your bench. In addition, you need to relax your back muscles (shoulders relaxed, i.e down). Practically every such problem is related to unnecessary tension in some muscles.
Get the book.
