A sign on the door saying "baby sleeping, don't knock" will only make it worse, I think. Imagine their glee at the thought they can wake the baby and cost you and their teacher some sleep.
I agree with those posting to suggest to do motion sensor lights. Get very bright ones. You can mount them on all sides of your house -- sounds like you need it, if they were on your deck, which by the way is trespassing.
Be certain your cars are parked where a motion sensor light would cover them too, in case the vandalism starts to extend to your cars. I also think cameras are a good idea.
If there is a way to rig "flashbulbs" that make it appear a photo's been taken when a motion sensor light goes off, all the better. That might drive them off, if they believe their photo's been taken. They likely already know that video cameras can be fakes, but a good flash in their faces could be a deterrent. I would ask a home security specialist company about this.
There is a much bigger issue here. Is the student body at this school so full of jerky kids with ignorant parents that this happens to other teachers too? I would ask your husband to find out from other teachers if this happens to them as well, and to have the school administration notify all parents, at the very start of the school year, that this has been going on and that teachers have agreed as a group they will all call the police about EVERY incident. It would also help if the school would have some form of discipline it will use whenever any student is caught by police doing these things. I know schools often are reluctant, or legally constrained from, punishing kids at school for things they do outside school, but....if this is an ongoing issue for multiple teachers and staffers, there must be something the school system would allow the school to do if a kid is caught. This is ridiculous. It's not pranking, it's vandalism, and it's time the kids knew that.