My students have short feet! Of course, this is because most of them are 10 years old or younger! When your foot is short and your teacher insists that you keep your heel on the floor, you have to lift your toes higher than an adult does to clear the pedal fully. Add to the problem the fact that my piano sits on casters making the pedals about 2.5 inches higher off of the floor than without them. The result? Muddy pedaling and/or hoe-down pedaling (entire foot stomps up and down, like at a hoe-down).
So, to equalize things, I've been allowing some of my students to do what I used to do when I was young and played the piano at my church. I stuck a hymnal under my heel. As a lover of books, I feel a little bad allowing a student to put their foot on one, but the hymnal is usually handy, and it's just about the right thickness to compensate for the casters. If you just can't bring yourself to defile a hymnal, you could use a block of wood. After a while, feet grow longer, muscles grow stronger, and the need for the hymnal goes away. Until then, this works for me!