Previous Homeowners haven't changed address with their kids school to avoid out of district fees

I have owned my home for 3 years this coming October and I have continually gotten their mail from various places. In the beginning I'd put it on my porch for them to grab since they still live in the area, but this is ridiculous.

Their daughter is going into first grade and got this cute postcard from her new teacher. I don't want the child to lack anything due to her parents so I tracked them down on Facebook and sent pictures of the postcard. They said they'd pick it up if I left it on the porch. These people are NOTORIOUS snoops and are desperate to see what I have done to the house so I closed all of my blinds.

I couldn't believe that they hadn't changed their address with the school since that seems important, but then someone pointed out the district fees to me. What steps should I take? Do I just return to sender? Call the school? Other? I don't want to just leave it alone because they have taken advantage on every inch I gave them.

Update: since a lot of people are assuming things about me. I am not trying to be nosy or tattle on anyone. I simply want them to stop sending stuff to my house, that I legally bought from them, so they can see what I did in a remodel. I have caught them on my video doorbell and pet cameras walking around the outside of the house looking in. It is personally really weird to me.

I know I am speculating about the district fees but given that they have done other shady stuff it wouldn't surprise me.

As many of you suggested I will be putting my name on my mailbox, getting a "Return to sender" stamp, and talking to my mail lady.

I don't want to be complacent with fraud either if they were to get caught. I have been throwing away other junk mail from them but recently I've been getting more important looking documents again (which I never open).

I have given these people 3 years of leeway and it's about time i get rid of their nosiness. I just didn't want to put their children in a bad spot. I am in a slightly better school district than the one they moved to so that is where the assumption came from.