I think it must be from top left to bottom right. Because: We read from left to right and we look from left to right and that makes it feel right if the arrow flys from left to right. It also think it should pointing from top to bottom because a arrow always falls a bit.
Shoot her in the heart with an arrow and then ask her opinion. I don't think there is a standard direction, as there seems to be images of it going in lots of directions, the most common being top left to bottom right or top right to bottom left.
Logically, if you want the arrow to have the traditional slant to it, it'd probably need to come from the top left. If it was aimed at top right, it'd have to pierce the sternum, which would be a far more difficult shot. Myabe theres more to the anatomy thing. Any idea what part of the heart is supposed to be stimulated by…
It should go front to back. Not likely to get thru the rib cage any other way :P I think it's BS that your teacher is even questioning the direction. If you're concerned about a grade and basing your art on what your teacher thinks, then just ask what s/he thinks the direction should be. It doesn't matter what we think is…