According to the recent (Spring 2023) IT survey, many members of Fearnhill staff are still using their PC and projector in "Duplicate" mode. This means that whatever is seen on the computer's screen is also shown (duplicated) on the projector screen.
There is a significant risk when using this duplicate mode, not least that if an alert - or your email - suddenly comes to the front focus, everyone in the classroom will be able to read it.
In extend mode, your desktop is extended over both screens so the projector is extra space. When you run PowerPoint, it will put the presentation on the projector and on your PC screen you will see "presenter view" that shows you the current and next slides. You can also check your email, do the register etc. without needing to freeze the projector screen.
To enter extend mode you just need to press and hold the Windows logo key and tap P. This will present the display options from which you can select "Extend".