http://www.channel3000.com/education/18055586/detail.html:

According to Dr. Mike Kuehne, the principal of Craig High School, the reasons for keeping students has to do with the fact that some students who leave for lunch don’t come back.
“What we’re really trying to do is look out for the best interest of our students,” Kuehne said.
Safety is another concern for Kuehne. School officials said that they can’t control who visits students off school property.
“They hang around young adults — 18-, 19-, 20-year-olds — many of them not from our community and they’re just hanging there to associate with the kids who are going to lunch,” Kuehne said. “It’s not an environment that we think is safe for some of our students.”
So far, if the campus is closed next year it would only affect incoming students.