I'm going to just touch on the starting early. I have a son (they lag a little behind girls), even though he was well-within the boundaries of the cut off - he was a fourth of July baby - he had a hard time due to his height. He looked at least two years older than he was. Everyone expected more of him. He graduates this year. He will only be 17 when he graduates.
I really want to address your concerns about the teachers in your childn's school. Yes, the kids are assigned to teachers. However, you can still have a say. Whenever, there are school events, make it a point to attend them and get to know the parents and teachers.
You will find some parents like teacher X, other parents despise her. Find out teacher X's teaching style. If that is something that works with your child great, no worries. But if you are truly concerned there are two things you can do.
1. Writer a letter describing a teaching style that works best for your child. Also, note the classroom environment, "A rigid classroom with a stern focus is not best for my child. She shuts down in such an environment and it would be detrimental to her education." NEVER, say what teacher you want. Schools are not fond of that. ONLY say the name of the teacher you are trying to avoid if you have no other choice, "We know it's out of the school's policy to pick teachers. We only have a concern with teacher X." To do this, you need to have first hand knowledge of Teacher X. Not hearsay. For example, my oldest son had a teacher that was horrible for him (parents either loved her or hated her...no inbetween). When my twins came upon their fourth grade year I wrote a letter outlining the teacher's style and what style was best for each twin. I mentioned the teacher's name that I wished to avoid, not just because of the twins' learning styles, but because I felt she had never addressed my concerns when my oldest son was in class and we butted heads constantly. My twins were assigned to different teachers (whew!).
2. The next option you have is to be the squeaky wheel. If your child is in a classroom with a teacher who is truly wrong for her, speak up. Start with the teacher (always log who you spoke with, the concerns you shared, and whether those concerns were addressed or not), move to the principal, then the school superintendent. Write letters, emails, and make sure they are dated. Enough pushing will get the situation taken care of. Just make sure your claims are legitimate. Sometimes we don't always like a particular teacher, but they may doing okay with our child. You can't have the best teacher every year, so make sure it's really an issue.
Sorry, that was long, but I wanted to address the teacher concern. Hang in there, and stay involved. It really helps.