How to create a simple circuit with a torchlight, 2 AA batteries and a voltmeter?
Welcome to the site and pleased to see that you're experimenting with electronics and electrical circuits.

It looks like your batteries are stacked one on the top of the other when they're inside the torch. If you could get two wires connected to the top of that battery stack and to the bottom, you could take them to your multimeter.

Then, as the batteries discharge into the light bulb, you can monitor the battery voltage and see it get lower and lower as the batteries go flat.

If you just push the wires into the space between the battery and the battery holder, they'll keep falling out so that's not the way forward. If you could scrounge some small strips of copper from school or buy some, you can solder a wire to each. Then you can push the copper in between the battery and holder at the top and bottom and connect those to your multimeter's voltage input terminals.

An AA battery is around 14 mm in diameter, so your copper strips would need to be 10-14 mm wide and maybe 40-50 mm long. Check carefully before using a different metal because it may have a higher resistance that'll affect your experiment and maybe spoil it.

I don't know what you have to hand but it's what I'd try and do. You can then easily move it from torch to torch.