Twelve Year Olds Strike Construction Site!

Last night about 9:00 pm four children about 12 years old crossed the School 18 construction fence toinspect the site. My eagle eyed 18 year old noted them and proposed to call the cops. Calmer parental thinking called for a casual walk out side with a flash light to advise the youngsters to be careful and not to get hurt. It appeared that it was an ordinary childhood exercise that any of us might have done. They then left after walking through and viewing the site. Aside from routing curious youth out of the area what can we do to deter such behavior in the future?