something like this is usually too trivial to mention in a tennis instruction but I find it very important just like how important it is to very precisely control the wrist and hand in manipulating a racquet. foot angle and force distribution are something that can be controlled consciously relatively easily with good results esp people who don't do it automatically from their own coordination. with some right training the foot muscles can get a lot stronger and the flexibility can be improved as well, which almost always escapes the mind of people for its trivialness.
as for your students, how effectively do they use their lower back and abdomen muscles to generate power? I'd imagine better than regular players.