This app was mentioned in 1 comment, with an average of 3.00 upvotes
I am one of the developers of a very simple scoring app which you can find here: https://play.google.com/store/apps/details?id=nz.co.redblue.volleyball which might suit.
Some guys in Portugal are using it to do something similar in a stadium by using Andy Emulator on a computer attached to the screen and using an Android device to control it. You could also cast to the screen using a chrome cast or the build in Android casting if the screen supports it.
It's simply a web page that runs in an android web view so I could make it available online or send you the html file if that's easier.