The school ratings are just as grades are. A is the best and F is the worst. Having said that, when you look at the numbers, you can see that there are many B schools that are just shy of an A and even C schools that are very close to a B.
The best way to gain understanding about a school is to visit it and talk with the staff, and then chat with some parents living in the neighborhood where you are planning to live. School Grade is important, but it doesn't tell you everything about a school.
There are also magnet schools and charter schools that are not a 'zoned school' but are one alternative if the zoned school does not seem up to your standards.
Hope this was helpful to you.
Coldwell Banker Vanguard