They built a new high school when I was in 3rd grade and converted the then-high school into a 'middle grades'. They have also since added a junior high.

grade school was brought into meaning because schools did not have grades. They had readers and you just did whatever you'd learned and kept up. As they got bigger and had more students to manage, the schools were divided up into grades so that everyone went at the same rate. thus, a 'grade school'.

If you'd like, I can add 'middle school' and 'elementary' and 'junior high' to your poll.