We just had our Relay this past weekend and we used the wrist bands for the teens.  We've used them for the last few years and it works great.  We have the team captain/chaperone put their cell phone number on the band, which is normally yellow, so we can contact them if we need them.  The yellow bands make it easy to spot the teens that are not authorized to be there.  If we catch them without a band and they can not produce one we escourt them to the exit and watch them leave.  Be sure everyone on the teams knows the rules.  Our teens must raise $100 to spend the night and they must be registered by 6 pm that night or at 10 o'clock their parents pick them up.