Is it permissible for my wife to work as a guidance counselor in a mixed-gender school, knowing that she has her own office and only occasionally deals with the institution's director?

If the wife's work exposes her to seclusion (khalwa) or suspicious mixing with non-mahram men, then it is impermissible. However, if there is no fear of temptation for her and there is no seclusion or suspicious mixing, then her work is permissible. Nevertheless, it is preferable for a woman to work in a place where she does not mix with non-mahram men, due to the harms that usually result from such mixing.