I told them to try and avoid stereotypes and to be "sensitive," keeping in mind they're reading theirs aloud for a mixed audience. It was great! The students were respectful and funny and insightful.
I started out by telling them about my story (I've actually blogged about it once before) -- how I assumed a male narrator for my last novel, and how challenging I thought it would be. Turns out, it wasn't that challenging at all, because I stopped looking at my character as MALE first, and started looking at him as HUMAN first. And it worked. I thought of men that I know personally - father, brother, cousins, friends - and tried to blend in realistic character traits that a man would have. Sure, my character liked sports. And sure, he wasn't very big on sharing his emotions. But that truly was part of his character, less than because he was a "man."
Today, I encouraged my students to challenge themselves - to write out of the box, wriggle out of their comfort zones. To consider writing from a different point of view - the opposite sex, or someone much older, much younger. I still think it's possible for writers to connect with characters much different from themselves. Because, when it all comes down to it, we're all human. Young, old, male, female. There are just certain things about human nature that we all share, that will never change.