Hello Matthew
I think I've found the cause of this problem to be MySQL being run in strict mode.
The install script is trying to insert a nearly-empty record to begin with (with the intention to update later), but the table needs certain fields to be completed. Some installations of MySQL will allow this to proceed, while some won't.
My suggestion for you, is to update the classroombookings.sql file in the root of the classroombookings folder with a fixed version at this URL:
You can either change just the highlighted section of SQL statements in your file with the ones in the above link, or replace the whole file - whichever is going to be easier for you. Then run the installation page again by visiting classroombookings/index.php/install.
Hope that works for you.
Craig.