How to display data with variation to unsophisticated users?

In my application, students are grouped into classes and each student has a performance metric. I want to be able to show teachers and principals a list of classes, along with an aggregate performance metric for each class.

I don't want to show the mean, because one very high or low performing student skews this. I don't want to show the median, because this basically discards the very high or low performing students entirely, and I want to capture the contribution of these students.

More correct would probably be to show a visualization of the mean with standard deviation. Conceivably I could show a little bell curve sparkline to convey this. But even then, I think this would be too sophisticated for many of my users, and it's not a single number that could be used, for instance, to sort the list by "performance".

Is there a better technique for computing aggregate performance into a single number that somehow captures the variance of the data set?