Our school keeps the gates open for people around the neighborhood to use the track to run, the field to walk their dogs and family activities, etc. It's a public school that keeps their gates open 24/7. It also attracts a lot of homeless people and the school is already used to that. I even remember 30 minutes after school ended my friend and I were walking around after futbol practice and we saw some homeless dude walkinga round with a trash bag.
TL:DR School is open 24/7 for the community, school is used to homeless, they won't care if they detect motion in the middle of the night.