Maybe a slightly different way to say what everyone else has is this: a letter from a student would not be seen as a letter of recommendation. Of course, it might literally be a letter which recommends you, but it simply wouldn't be seen as one. If you actually have a student who you think will write you a good letter, that would probably be nice supplementary material for your application, but it shouldn't be included in the list of letters on, say, mathjobs.