I'd caution against relying on priesthood holders to provide security at activities. Instead, all adults should be educated on how to provide security for themselves and for youth.
All adults should know basic safety principles, such as observing the parking lot before exiting the building; knowing where the publicly available phone in the building is; understanding that emergency phone numbers are posted next to that phone; and they should all be informed that if something seems amiss on the church property, they should notify the police.
Recently in my ward, the Relief Society was pushing very hard to have a priesthood presence at their activities citing "some policy they once heard". The bishopric told them that it wasn't necessary but could be arranged if they wanted. The Relief Society insisted that it must be done because on more than one occasion in the previous two years, someone had approached sisters leaving an activity and flashed the sisters in the parking lot. What baffled the bishopric was that no one had notified the bishopric and, more disturbing, no one had notified the police.
We have made sure a priesthood holder is present at each of their activities since then, but that was mostly to appease the Relief Society presidency. The more important steps we took were to write out safety recommendations and policies that the women could implement themselves and then having a detective from the local police force come speak at a Relief Society meeting about personal safety. At the end of the day, the message we wanted them to get was that the primary line of defense for any person's safety is their own education, preparedness, and awareness. Having established that principle, we were happy to have the priesthood help out, but we were clear that priesthood holders were not and should not be the primary defense.