There may be restraining orders and the issue becomes a legal battle for the school. Custodial parents may often fully expect the school to be responsible for court knowledge that often is not disseminated to everyone on campus. Most campus supervisors and security staff work alongside each other in dealing with confrontational parents on campus. Radio systems are the norm at most campuses whereby other help can be alerted. Often, security will tell you it is quite useful to call several people when approaching a suspected angry parent.
Teachers often can alert security about a possible confrontation.
Security personnel can conveniently check-in to ensure that things have not escalated to a confrontational level without the support of security or administrative help for the teacher or support staff on campus. Security personnel are the peacekeeping force on a campus and generally, most confrontational parents will support that. Front office secretaries are often the first to confront oppositional parents. Instructional assistants and special education aides are another group that may come in contact with oppositional parents. Rumors, inaccurate and confidential conversations can provide additional agitation to an already oppositional parent.