High school is a kind of school, a place where people go to learn skills for future jobs. Most people go to high school right after middle school. Most finish high school but some people stop in the middle or choose to finish high school later.
In the United States, a high school is a school that students go to usually for grades 9 through 12, from the ages of 14 to 18. It is also the last school that the law requires the student to go to. However, students with behavioral issues that are too much for the school to handle are not required to attend. This is called expulsion. Some people may choose to take some tests called GED (General Education Development) as another way to finish high school.
There are public high schools and private high schools in the United States and the United Kingdom, and many other countries.