Complex sentence: Students at Montclair High school can all leave the campus for lunch and free periods, by an open campus policy, however due to a incident caused by students of the class of 2012, Freshman are no longer able to leave Rand park for lunch.
Split into simpler sentences: Students at Montclair High school can all leave the campus for lunch and free periods, by an open campus policy. However, due to an incident caused by students of the class of 2012, Freshmen are no longer able to leave Rand park for lunch.