You may already have a smartphone, even one without any paid connection to phone or internet (perhaps an old, retired one). Put it on a tripod in front of you, camera aimed at the fretboard, landscape not portrait. Ensure that its screen shows what you want. Then transfer that display to your computer and thus to the classroom's projector. This last step has many informal names that change from month to month:
- mirror my Android screen to my PC
- project my mobile screen
- share my phone screen
- screen mirror my phone
- cast (i.e., broadcast) my mobile screen
The technical details of how to do this also change from month to month, and thus must be left as an exercise for the reader.
The projector screen should be pretty large, larger than the real-life fretboard, unless the camera so close that it sees only a few frets.