The bag the detective uses for the necklace is not a police evidence bag, it is a property bag such as is used at a jail.
At the beginning, Avery states Mandy "5 years ago, she moved here from San Francisco." Yet, a few minutes later Kaley says "This is our last year together before we all go off to college. We've been friends since the third grade" So they are in 12th grade. Now if Mandy moved here "5 years ago", that means she didn't meet them until 7th grade.
The girls are pranking John, but never take a photo of him holding the sign he made. They instead only took photos from behind which do not identify him or show his sign. Popular prank sites that expose Nigerian scammers always have the pranked subject holding their sign.