Visible crew/equipment: The scene where Spencer throws the boomerang for the first time and misses the bikers they take off running. While they are running from the bikers, there is a shot right before the boomerang hits the bikers where you can see a camera man sitting in the bushes on the left side. You don't have to pause it, but can to see it better. (00:36:30)
Visible crew/equipment: When the girl is dragged by the motorcycle, she unties herself and starts to walk. A crew member can plainly be seen standing in the background. (01:35:00)
Continuity mistake: When Martha approaches the men at the transportation shed, the stereo is visible on the bench on the right and its front is parallel to the side of the bench, but when it switches on for the dance fight, it is now at about a 30 degree angle, touching both sides of the bench. (01:10:30 - 01:12:00)
Answer: They still had detention. The only thing that changed was Alex. But since they had become such close friends, detention would hardly be a punishment for them anymore.
It just seemed like they just simply walked out of detention. I mean did they finish their detention or did they had to continue on a Saturday?
The movie doesn't explain. But regardless, it also really doesn't matter.
They probably didn't go back on Saturday. When they go back to school, Spencer acts like he hasn't spoken to Martha since their adventure, while Bethany says she's been texting Martha 'all weekend'.