When you're dismayed, you're aghast, or shocked by disappointment. Your dismayed teacher's face showed that she expected the test grades to be much higher.
Dismayed describes feeling appalled or distressed. The word implies a sense of being let down, the way you'd feel if your favorite baseball team lost the World Series, or if your birthday present wasn't the ticket to France you'd hoped for, but was just a pink sweater. You can also feel dismayed when expecting more from someone, like being dismayed to see your environmental science teacher throwing recyclables in the trash.