Wow! Super sorry you had to deal with that in a class. As with all things, there is some variation to how long people’s arms are, although we can create an “average” or “ideal” proportion measure. 

I’m sure you’ve seen something like this before (This is by Andrew Loomis, who is an incredible staple in art classes I’ve taken): 

I *personally* think this is a bit long (and your teacher’s measure of the hands resting at the hips a bit short), but that is personal perception. Let’s look at some studies.

This also isn’t meant to be a hard and fast rule, of course. Guidelines can be tweaked to a person’s personal preference and seeing-eye. One tidbit to note is that in taller people (with longer legs), the tips of the fingers may stop a big higher than halfway down the thigh, because their legs are so long. 

As you can see in this last diagram, the fingertips lay almost a hand-length away from where I put the knee. Perhaps the disparity between how I draw arms and how you draw arms isn’t within the arm at all, but the proportion of the rest of the body (ex. if you’re drawing the legs or torso longer/shorter). Just something to think about! 

-Mod Future (ko-fi)