Could you send a reply politely fishing for more information? Something like, "It sounds like this might take longer than a normal 15 minute p/t conference. Should I block out more time in my schedule?... I'm happy to talk to the "team." Can you give me an idea why everyone has been invited? DH is out of town and if it's outside the normal p/t conference information, we might need to set up another meeting with everyone so he can be present too."

Maybe the teacher will give you some insights to help you make a better decision... or maybe just dodge your questions. But if you're polite, I don't think it will hurt.