Don't ask, and they may tell
Never look 'em in the eye.
After quite a few years of child-rearing, that's the sum total of my advice for any parent trying to elicit information about the school day.
Modern communication may be all about the bold voice, firm handshake, piercing eye contact. "Alert, poised, and confident!" as my stepfather used to say when mindlessly asked, "How are you?"
Forget this with kids. Think (apparent) disinterest. Think statements, not questions. If you want really good stuff - especially from boys - think exercise.
Some of the best conversations I've had with my son have occurred as I jogged and he bladed. One not-inconsequential factor is that I (by necessity) say less and less as our outing progresses. But more to the point is that we almost never look at each other - nor are we side by side the entire time. And he is blissfully mobile.
This approach gives us a certain ability to control the dial - change the station easily, if needed. Mention a friend, a teacher's odd habit - and we chug steadily along. Toss out "homework" or a girl's name, and suddenly I'm straining at words that trail behind his disappearing back.
We once had a lengthy discussion about his eighth-grade sex-ed class this way. He would have submitted to torture rather than discuss this face to face. But running? Something about a house we passed brought it to Matt's mind. Suddenly, he made a comment or two, then time out. Another observation - then he'd whiz off, turn, yell something with amazing abandon, zoom by again.
The topic melted away as soon as we got to our driveway. But I knew enough. Now we could turn our attention to what to eat for dinner.