The literal mortar board is the wooden plate, usually with a handle underneath, on which bricklayers carry small amounts of mortar. A similar tool is used by plasterers, but they usually call it a hawk. The academic cap is often also called a mortarboard
Because the academic elite want to continue their lines and the PhD hats are SO much cooler than the stupid mortarboards, so there is built in incentinve to continue one's education and get the better costume.
High school commencement refers to the formal ceremony surrounding graduation. It is usually held in a large public space, such as an auditorium, before an audience of student friends and family members. Commencement includes speeches by school officials, a keynote address and speeches by selected students, including the class valedictorian. Faculty usually dress in their academic robes and mortarboard hats, and graduates dress in similar robes/gowns and motarboard hats rented for the occassion. The highlight of commencement is the awarding of diplomas to the graudates.