I've surprised my DD twice now. The first time was when she was 6. We had to be to the airport relatively early so I woke her up with When You Wish Upon a Star playing and told her to get dressed, we had to get to the airport. The second surprise was just a few weeks ago. This one was closer to your situation. We were going down for a short weekend to see the Osborne lights and go to
MVMCP. I picked her up right at the end of school, which was early for her because she usually goes to the after-school program. I had told her that morning that I'd be picking her up early because "we had some Christmas stuff to do." I told her where we were going when we pulled off at the airport exit and she finally demanded to know where we were going.
If I were you, I'd wait until you pick them up from school to tell them. Otherwise, they might be too excited to concentrate at school. Do they have special carry on bags they only use when they fly? You could hand those to them when they get to the car in the school parking lot. Maybe hand them their boarding passes?