Why would you install a single 48-port fixed-configuration switch, rather than two 24-port fixed-configuration switches, in a wiring closet that supports two classrooms?

Fixed configuration switches contain a fixed number of ports and cannot be expanded.

Installing a single 48- port fixed-configuration switch is better in two ways. First, the switch contains 48 ports which are more than the other fixed configuration switch containing 24 ports so there is an availability of more ports for source and destination devices. Port is basically an interface between computer and other end devices. Its advantage is that more devices can interact or connect this way, with less packet collision, hence enhancing network performance.  

Secondly rather than setting up two switches, which will take space and consume more power, its is a better way is to have one switch installed. It does not require much space. A single 48-port fixed-configuration switch will have less space and power requirements as compared to two 24-port fixed-configuration switches which is more suitable for supporting two classrooms.