YOu know what I'd do? Follow it. I'm not even kidding. After you drop your kiddos off, just see where it goes and what it does. Maybe it's a parent who is just a little sad and nervous about leaving her baby? I watch my daughter play at recess from the parking lot sometimes; perhaps this car's driver is doing the same?
I don't think it'd hurt at all to write down the license plate number and inform the school. Isn't it possible to find out who's registered to a particular car? If so, then you can try to match up the name with the sex offender registry. But then again, it could just be someone driving his mother's car, ya know?
We all have momma instincts...follow yours!
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To clarify, I don't think you should go all Green Beret on the person, or anything. But no, it wouldn't hurt to just see where it goes down the street. Don't break out your binoculars or follow it around town all day, making notes or walkie-talkie-ing info to your girlfriends but I think it'd be perfectly okay to just keep a safe distance and see what it does.