I am a mandarin native speaker. The sentence confuses me as well. “表示” is followed by “了”,which means THE TEACHER EXPRESSED CONCERN. But we can't tell if the test have been given or not just by “考试太难” .
Besides,we don't say “老师表示了担心考试太难”,it's too long,which needs a comma to stop,we say “老师说了,他/她担心考试太难” 。“表示” is far too formal,usually used in news、reports、papers.