When Kathy is witnessing the murder, in the first half of the scene, her shirt is tucked in, but during the second half when she runs away and asks Ellen and Carla what happened, her shirt is untucked.
We don't know who held down Missy's face during the actual murder, but in the movie, it is shown that Carla is holding her head and Ellen is holding her legs.
Missy Avila aka Jenny actually had 3 brothers rather than 2 as it is shown in the movie.
Jenny's face goes from being dirty and muddy from Ellen smearing dirt in her face , to being completely clean before Carla shoves her face in the water , back to being dirty when she manages to come up briefly for air .
When Jenny is waiting for Carla on the porch, she takes a purple seashell mirror from her purse. When Carla comes to pick her up, she puts the mirror in the purse. As she gets into Carla's car she is carrying the purse with her--during the death scene, when Jenny gets out of the car her purse is still in Carla's car. But when Jean is given the clothes and items Jenny was wearing during her death, her purse isn't visible either meaning it is still in Carla's car. However, later on when Jean and Cheryl are cleaning out Jenny's room, Cheryl picks up the purple sea shell mirror out of Jenny's makeup basket. It couldn't possibly have been in the basket if it was still in the purse, which Jean doesn't have.