The photo Jack examines, which shows Helen kneeling to pick up the compact, doesn't match the position she assumed earlier when it was actually shown.
When Helen closes the No Admittance door, you can see the reflection of a crew member in the glass.
Helen is helping decorate the gym for prom, but earlier she dismissed the entire prom as nothing.
However, when she dismissed it, she was dealing with a boy that was only asking her so he and his friends can make fun of her-she was simply being sarcastic. It was also just before she discovers the compact's power and by the time she helps decorate, she's already formed the plan to steal Scott from her sister and go to the prom. Barring that, though, it's not out of the realm of possibility that she'd still help even if she wasn't interested in going.
It's night at the school, and apparently closed for everything except prom decoration. But when Ryan follows Helen and Russell, he bumps into two students casually standing at a locker. The prom is shown as happening the same night, which isn't how American/Canadian proms work. [When Ryan follows Helen and Russell, it's during the day. After killing him and dealing with Ryan, Helen goes home and gets ready for the prom. They were setting up the deco during the day, Ryan carrying around a book bag suggests classes may have been in session. The two students may have been on a free period. Even if it was Saturday those two students could simply be taking a quick break from decorating and having a chat where they wouldn't be in the way. This could explain why everyone was so casually dressed.]
Helen and Joanne's parents are never seen or referenced, even though both their daughters are at home and going off to prom.