Quote:
Originally Posted by Clark What if I extend the sentence?
She was one of the teachers who were selected for the contest.
Still no? |
Nope. Sorry.

The subject complement is singular, not plural.
Back to the poster's example, the answer is always in the question. Note that, assumption and ellipsis are the missing links here:
Ex: She was among (one of) the teachers who were selected for the contest. 
The poster's example, however, does not provide those missing links. Thus, we deal with what we have:
Ex: She was one of the teachers who was selected.
Meaning => She was a teacher who was selected.
Ex: She was one of the teachers who were selected. 
Meaning => She was a teacher who were selected.