The conclusion of the cross country race shows all of the runners past the finish line within a few seconds of the winner, a result that does not match how the race was shown in its progress i.e. the runners would have been more spread out at the end.
During the cross country race, many of the runners are shown in a constant near sprint, a pace none of them would be able to maintain over a cross country race.
Near the end, when Greg's mother and Reed are in Reed's car, it's obvious the windshield has been removed or modified to eliminate the camera's reflection.
When all the counselors save Greg on the cliff, unless there are additional adult personnel present not shown, no one is supervising the rest of the camp of troubled teens.