School Ratings - are they stable?

[Phil Monkhouse]

I'm moving from Canada to SW Houston soon and was wondering if you can count on school ratings (greatschools.org) to stay consistent or do they rise and fall? If so over what time period? If you buy in a neighborhood that you expect to rise in value but has poor school ratings now can you expect the schools to get better as the rest of the neighborhood rises in value? 

It's very important for me to buy in an area with really good ratings for my personal house, is that true for every family? Very different in Ontario where you can generally say public schools are the same no matter what area.