I know that the word "staffroom" is a common word used in the UK schools for "a room in a school that is for the use of the teachers when they are not teaching". But is it the same in American English? No alternatives?
The term staffroom appears to be mainly used in BrE. The more commmon term in AmE is teacher’s lounge:
A staffroom, also known as a teachers' lounge outside Great Britain, is a room in a primary school, middle school, high school or college where teachers have their desk and prepare their lessons,
as opposed to the faculty lounge in United States and Canadian universities and to the common room in British independent schools, where the teachers and/or school staff talk to each other, discuss work, eat, drink and socialise while not in class. In primary school, there is usually just one common room, while in middle school, high school, college and/or university, there can be multiple common rooms. Faculty lounges and common rooms may have kitchens, desks, and television sets.