Continuity mistake: When Henry & Mark were chased by the dog, look carefully at the back of Henry's jeans and you will see a large dirt stain on it, but in the following scene over the cliff, there is a brief overhead shot over Henry and you will see that stain is gone.
Continuity mistake: When Henry attacks Connie for coming into his room and Mark steps in to stop him, they start fighting and Connie runs downstairs to tell on them. As the shot alternates back and forth, Henry's right hand jumps from Mark's ear to grabbing a handful of his hair and then back down again, but we don't see either boy move in a way that would explain the switch.
Answer: Yes. She had to drop out when production was delayed, and she was no longer available.