You create an amalgamation by taking separate things and combining them into one. If your school is closing and joining with another school to create a new school, that is an amalgamation.
Commonly, amalgamation is a term used to describe the merger of two or more companies into one. Before J.P Morgan Chase was a single bank (or an amalgamation), it was two separate entities: J.P. Morgan and the Chase Manhattan Corporation. When you want to describe something as being made up of many previously separate parts, you can say it's an amalgamation.