Wordsmyth seems like an exceptional online dictionary that lets you create several different types of vocabulary quizzes. Teachers can get accounts for free. The site has many other features, as well.
I’m adding it to The Best Ways To Create Online Tests.
However, Bob Parks (its creator) tells me that they “are developing new functionality for teachers, including a full vocabulary study system.” When that happens, I might also add it to The Best Sites Where ELL’s Can Learn Vocabulary.