Here's a study that shows that some people with high functioning autism have more trouble matching up similar emotional expressions (perceptual matching), than labeling with words. This may explain why some HFA students seem to be able to correctly answer emotional expression questions in practice, but struggle in real-life situations.
We have to be careful about thinking that students 'know' by how they've responded to us in words. Nonverbal practice should be interactive with frequent feedback to them. This can be done like drama practice, impersonations, or charades. Practice with a mirror may help, and verbal labeling and exaggeration can improve visual recognition.