Which is why you’d get an external flash (speedlight).

You probably got your answer by now lol but just in case you didn’t I’ll say it anyway. Yes, you still need it because, for instance, in the semi-automatic mode Aperture Priority, as soon as you recompose your shot, wherever your camera ends up pointing is what your camera will take its meter reading from when you press your shutter button all the way, not the subject you were initially focusing on.

Engaging Auto Exposure Lock before recomposing will ensure that your camera retains its initial meter reading upon establishing focus on your subject.

I use an old Canon EOS 60D DSLR. I have my camera in the configuration where my AF-ON button is used for back button focusing and my shutter button’s half press function is used to AEL instead of the dedicated AEL button (which I don’t ever use for anything lol). I find that works best for me in Aperture Priority.

Double exposure and post-production

Forgive my beginner-level question! In your reply to Lara, you say “I used the Exposure Lock button to lock exposure on the students,…” Well, do you imply that when the Exposure Lock button is pressed what takes place is the camera quickly prepares itself for an ideal exposure with the right aperture and shutter speed, and holds on to these as long as the EL button remains pressed? In other words, can the EL button bring into effect an ideal lighting for the shot planned?