Get his license plate number, without him knowing. Look on his vehicle.
Background check him.
Alert the SCHOOL, that this is going on.
He is FRAUDULENTLY.... claiming to be a parent.
AND, who picks her up from school after school? How does she get home? Are any of you home when she gets home? Or is she just by herself?
Lots of loopholes/opportunities for her and that guy, to get together.
DOCUMENT everything.
Look on your Niece's cell phone, get his phone number too.
Take PHOTOS of him if you see him, with your cell phone etc. And if you have a photo of him or his vehicle etc., it is also documentation and you can use it to show the Police etc. if anything happens.
I would also, take a CURRENT photo of your Niece, in case she is missing and you have to locate her.
Hopefully, he is not some weirdo, "grooming" her for untoward things. Like child trafficking.
Maybe he is not even the age he claims to be.
HOW OLD is he, actually?
You need to find out about him.
Don't tell him.
And background check him, or call the Cops for anything you can.
And I really hope you have told her Dad/parents about this.
It is very, full of red flags.
Him giving her a pre-paid cell phone, is very weird.
It means, that he is giving her that so he can't be tracked etc.
It is deception... on many levels.
And he seems to know, what he is doing. Deception, manipulating her to hide things and keep secrets, avoiding the means of getting caught, etc.
And, what the heck happened, when your Wife spoke to him, as you said?
Did she speak to him in person or on a phone?
That would have been your/your Wife's opportunity, to find out about him.
But of course, he probably lies... about everything per his identity.