High school is a kind of school, a place where people go to learn skills for future jobs. In a three-part system such as in the United States, children go to high school after middle school ("junior high"). In a two-part system such as in the United Kingdom, the change is from primary school to secondary school at 11 years of age.
In the United States, a high school is a school that students go to usually for grades 9 through 12, from the ages of about 13-15 to about 17-19. 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. A student is allowed by law to stop going to high school after a certain age without having graduated. In most this is 16, however in a few states it is 18 or higher, for example Wisconsin.
There are public high schools and private high schools in the United States and many other countries.