A 2018 THOMAS BUILT TYPE C SCHOOL BUS WAS TRAVELING AT A SPEED OF APPROXIMATELY 42 MPH ON A CITY/URBAN STREET WITH 5 PASSENGERS, PLUS THE OPERATOR ON BOARD. A TREE FELL AND STRUCK THE BUS NEAR, OR ON, THE RIGHT FRONT WINDSHIELD/ROOF AREA. THIS IMPACT CAUSED THE ENTIRE ROOF AREA TO COLLAPSE WITH SEVERE DAMAGE THROUGHOUT THE ENTIRE LENGTH OF THE BUS.