If my son comes in the house visibly upset, or if he says he's had a bad day, I'll ask him what happened, and when it's something that happened in the last hour or so, we'll talk about that (a long talk about something like this, for him, is about two minutes). Then I'll ask him if anything good happened. Usually, after he's "cleared out" the bad thing, he can tell me something good.
When he was a bit younger, around the age of OP's, instead of asking how his day was, I used to say - tell me something good and something bad that happened at school today. Sometimes the only good thing might have been seeing his friend during recess; sometimes there wasn't really anything bad. but setting the stage for allowing good and bad meant both were "allowed" and expected, and we could kind of celebrate the good thing and commiserate about the bad thing. We could also try to figure out strategies for dealing with the bad thing, if it were appropriate to do so (e.g. bad social interaction). Since it was a regular question, it didn't catch my son by surprise, although sometimes he'd mutter, "I don't want to talk right now." Usually that was okay with me, as long as it was understood (out loud, specified) that we would talk later, at a specified time, like after snack.
He's a teenager, now. It's gotten harder!
And kip, it is kind of a trick question - not to tear apart the "ok," but to get a sense of what's going on with one's child during the hours s/he's away! And not to invade privacy, but to know whether one's child is happy or not, and whether there are things that, as the parent, one could help with. Also, if there are things to celebrate, parents want to know! My son, for instance, did not tell me he was getting a math award - his teacher had to call and tell me that! Parents don't want to feel entirely cut out of their children's lives as soon as they start school, after all.