Since 2018, California law requires that all new commercial buses be equipped with seatbelts. California Senate Bill 20 (passed in 2017) requires all passengers over the age of 8 on a commercial bus to wear a seatbelt. Failing to do so is punishable by a $20 ticket.
Additionally, California Vehicle Code Section 27316 specifies that all school buses purchased or leased after 2005 must be equipped with seat belts.