That's true.
But this is one of the big reasons why I think Missy was killed inside the church somewhere, perhaps in one of those rooms that the killer opened the door to while he/she was wandering around waiting.
The class was typically held outside. It has been reported that, that morning or day before, Missy posted on Facebook that the class would be held inside due to weather. The class was not usually held inside, therefore I don't think Missy posted an exact room/area. I think it was just sort of assumed that, if the class was held inside, the campers congregated in the lobby part (by the entrance) of the church and waited for Missy then went together to wherever they were going to work out. The church is big...if everyone met up front and went together it would prevent anyone arriving a little later from not knowing where to go (since, once again, it was not typical for it to be inside).
ANYWAYS..so I think the first camper(s) arrived and waited around in that lobby area/front of the church (maybe even outside in the covered area) for Missy. They likely saw her car, figured she was there getting everything set up and would join them closer to the start time. When it got closer to 5 (4:55-ish maybe?) someone went to find her, called out for her, wandered around the church and then found her in one of those rooms and proceeded to call 911.
That's my theory for the 30 minute (approx) time gap.