In the United States, there's no answer. We use both kinds of school and you can't predict which will be used in any given town. In the U.S. there is an enormous amount of local control in school systems, and many things vary, even from town to town within a single state. Between "elementary school" and "high school" (or "senior high school") there will either be a "junior high school" or a "middle school." "Middle school" might be 6th, 7th, and 8th grades. "Junior high school" might be 7th and 8th grade, or 7th, 8th and 9th grades. Local school systems often change from one system to another, based on educational fashions, or school building sizes and demographics. People tend to use whatever name the local school uses, or whatever name their childhood school used. When listening, it doesn't matter--middle school and junior high school are roughly the same. When speaking, you just have to use one or the other. Middle schools are more popular, so it would be my first guess. If the person you're speaking to lives in a town that uses a junior high school instead, they will correct you. If someone calls the high school a "senior high school," then they probably have a "junior high school" rather than a "middle school."

It also depends on the time period. When I went to school, it was called "junior high" and had grades 7-9. Now its called "middle school" and has grades 6-8 or 7-8 in some schools.

It depends on the area you're location as to which term is used. They both mean the same thing so you're safe saying either. Hope this helps!

It depends on the country. All English-speaking countries have different education systems, and so have different terms to describe the stages of school. Which country were you thinking of?