I'm in high school = estoy inscrito en una escuela secundaria I'm at high school = estoy en la escuela, ahora, físcamente En grandes líneas, y sin que todo el mundo respete mis reglas. I work in public relations/computer programming/financial services I work at the X company
2 In South Africa (British colony till 1910 and ongoing strong influence till 1961) children in junior school (5 - 12 y old) and high school (13 - 18 years old) were referred to as "pupils". "Students" was only used to refer to people enrolled in university or some form of post-secondary (tertiary) education.
But what was the complete sentence? Specifically, what comes after "school"? I agree that if you are talking about the time when you were a high-school student, "in high school" or "at high school" is correct. That is, course, the most likely meaning. But many sentences could be concocted where a different preposition would be needed, for example:
I'm translating a document from English to Spanish that has many references to an American high school. It looks like the term "high school" in Spanish varies from country to country: Bachillerato (Most of South America and Spain?) Escuela secundaria (Some parts of South America?) Preparatoria...
In UK we don't have "high school" as a general type of school, although some have that in their name, for example "Stamford High School". So "high school graduate" is an AmE term. In UK students who are at secondary school "leave school". For example "Pete left school at age 18 with three A-levels". But "Peter graduated from Cambridge University with a First".