My friend is taking an English conversation class. In it, she said
I have been teaching math in high schools for more than 10 years. Before that I taught math in junior high schools.
However, her teacher corrected her, telling her she should change in junior high to to junior high schools.
Is there any reasons that we should use to? Isn't in correct?
As StoneyB states above, you teach in a building, in/at a school/institution. Just as you live in a house, eat in/at a restaurant, or sleep in a bedroom.
You cannot teach math to a place. You can't teach math to junior high schools, to houses, to restaurants, or to rooms. You can, however, teach math to junior high school students, to elementary school students, to high school students, to college students, or to anyone who wants to learn.
