If Sam is indeed hypoglycemic like Jack says he is at one point, he should not be taking insulin. Hypoglycemia is a constant running of low blood sugar and taking insulin would almost certainly kill him.
When the scarecrow attacks the kids while they are camping on the beach he punches a girl and misses by two feet and she still falls down.
When the two guys go to pee on the face of their friend who's buried in the sand, the camera is panned too low, revealing that no zippers are pulled down (bathing suits don't have zippers anyway!) and the two are holding the air.
When one of the girls is talking to the coach her microphone battery is clearly visible.
When the three surviving teens are speaking with the coach you can see their marks on the floor for where they are supposed to stand.
When Jack confronts his friends on the beach he says Sam is hypoglycemic first than says he is diabetic. These are two different medical problems.