What's much more interesting than "what not to listen to", from a learning point of view, since you are a classical piano student, would be, in the case of pop (or rock, metal, jazz) would be
Where are the similarities?
Where are the differencies?
Since popular music is thrown at us whichever way we turn our heads, we get sort of used to a certain sound which is probably very alien to the old 18th-19th century european composers. What springs to my mind is that pop-music has
*an unvaryingly steady beat
*more or less the same dynamic level throughout
which of course stems from it's roots in afro-american music. Don't forget though that the two worlds, afro-american and european-classical, met in the early 20th century, with Ravel calling the 2nd mvt of his violin sonata "Blues", and Stravinsky composing his "Piano Rag Music" among others. Cross-influences have continued ever since, but might be at an all-time low right now, or am I wrong?
(By the way, I'm not saying that afro-american music lacks dynamics!)
To play, and understand, Chopin and Beethoven, et al, is perhaps to try to forget some of our pop-music heritage, and try to get into another musical world, where rythm is flexible, and dynamic nuances are everything! Perhaps your teacher is, somewhat clumsily, trying to make this point.
We cannot, though, escape the influences of music from our own time. A funny example is that variation in the 2nd movement of Beethoven's last sonata, which make everyone jump: "That's jazz!", which is as anacronistic as it can be, but still we can't escape making that paralell..
Topic: Teacher says I can't listen to ANYTHING but classical