Can an asst.principal question a child for over an hour and ignore his request to call his parents?

Unfortunately, yes.  Even though children spend years learning about the justice system and principles like liberty and rights, the courts have rarely extended constitutional rights to children in the academic setting and instead have held that the schools have the right to engage in activities that are "reasonably related" to the administrative function of running the school.  The exception, however, would be if the asst. principal was asking questions at the request or as a favor to law enforcement.  Law enforcement cannot use other people to perform their function-- even under a different title like assistant principle.