Well, today I decided not to ask because I came up with a brilliant plan.
I asked the mother of another student if her daughter had been photographed, and she said yes, and it was for something the kids were doing that they couldn't talk about.
I know this woman's daughter, and the kid talks constantly, so I knew if anything had happened at school, she'd know about it.
This has set my mind at ease. I may still talk to the teacher about the surprise/secret thing, but I'm not actually worried, and I'll make sure that comes through when we do talk.
Thank you everyone for your help. I really appreciate it.