I think it is much better to put something under the bench than to put something on the bench. A selection of wooden boards or carpet samples large enough to accommodate the entire bench are good choices. Alternativey, having a set of benches that together cover the entire height range would of course work too.
Too many teachers may be sitting their students too high. Look at examples of Glenn Gould and Horowitz; they (and many other great pianists) sit very low. Children sitting on regular bench tend to be lower than adults, but not low enough to cause problems. There may be no need to raise the height.