Answer:
The main clause in the sentence Most of the teachers are Americans, though a few came from England and France is Most of the teachers are Americans, since it is the essential component of the whole sentence, made up of a subject and a predicate that jointly express a complete concept.
The subordinate clause though a few came from England and France is an adverb clause, because is acting as an adverb: it is adding information to the action in the main clause. In this case, the adverb clause is introduced by the subordinate conjunction though, that denotes contrast between the main clause and the subordinate one.
