“because her students did so well on the test” is a dependant and adverbial clause.
It is a dependant clause because it is part of a sentence, and to complete its meaning, it depends on the main clause (Miss Bottrell was beaming)
The subordinator conjunction “because” introduces the adverbial clause. It is a clause of purpose as it can be recognized by answering the question “Why was Miss Bottrell beaming”?
