Late 1990's.
Hearing teachers who use ASL when they teach should certainly be fluent in ASL. They should also be skilled in English use (sadly, they all are not).
Deaf teachers who use ASL should also be fluent in written English if any of the curriculum includes reading (text books, literature, student papers) or writing (preparing tests, sending notes home to parents, teaching English composition).
All teachers should have a well-rounded education and be fluent in their teaching language regardless of which "content" they specialize in.
IMO