Did you read the article? There are tapes, photographs, etc. And these kids were disabled.
You need to re read the article. The suit alleges the existence of such tapes and photographs and the boys in question were not disabled.
And I promise you that no child was allowed to leave any school campus with an adult without the permission of the parents. Public or private schools by that time had strict policies in place about who could or could not pick up kids. Heck even non custodial parents had to get put on the approved list. So either the guy did a good forgery of parental signatures or it never happened.