It really depends on what the school/teacher is able to do and what you expect from them. Some teachers/schools don't have the capacity to really teach the very advanced kids, so it doesn't matter that much whether they know that your kid needs a lot more challenge.
I tried to inform the K and 1st grade teachers about DS, following instructions in GT education books (taking your child's work at home to show the teachers, tell the teachers what books your child is reading at home, showing them IQ score, etc). But we've come to the conclusion that the school was not equipped with the right personnel and the right mindset to deal with very advanced kids. Yes, if someone is one grade level above classmates, the teachers are happy to do something. If someone is five grade levels above, they don't see it. So we pretty much stopped hoping for much from school since maybe 2nd grade, and we hardly had much communications with the teachers other than the once-a-year parent-teacher meetings which we politely sit through. And we never really talked with teachers/schools about our DD--we learned the lesson. For her, the goal is to be happy at school and we will handle academics at home.
I think you should also try to get to know other gifted kids and their families in your area (especially kids older than yours) to see what they are doing at school and at home. I think this is the best way to find out whether it's useful at all to talk to the teachers.
I almost wrote this post. This is exactly where our family is at (only, my DS is in 2nd grade now).
My best recommendation is to afterschool using the curriculum of your choice or enroll your child in appropriate enrichment classes or find the right tutors/coaches for your child.
