When you've done your best to explain what goes on inside a camera and you find that one person who has to be shown, show them in a Camera Obscura.
All you need is a room or space that can be completely darkened and one wall or side that can be exposed to bright sunlight.
Your classroom, especially if it has little or no windows, or a school auditorium can be your Camera Obscura if you're willing to do a little remodeling. You must be able to darken the room completely and a door must open on a bright sunlit area. Build a dark screen to cover the doorframe. Use roofing felt (tar paper) with strips taped or stapled together and mounted on a light frame which can be propped against the door frame on the inside wall.
In a spot on your screen which will be in bright sunlight (weather permitting) cut a hole in the tarpaper about an inch in diameter. Cover this hole with a patch of aluminum foil taped securely to the tarpaper.
Open the door(s) and place the screen over the door opening. Prop up the frame and tape as necessary to prevent light leaks. With your observers inside the camera, let the eyes adapt to the darkness for a few minutes. Find the foil patch on your dark screen and punch a small pinhole in the foil. Experiment to find the size, which will give the desired result.
You should see an inverted image of the sunlit area outside projected on the opposite wall of the room. If the walls are painted a dark shade or the image is not bright enough, place a portable projection screen opposite your pinhole.
To stimulate discussion, move the screen nearer to and farther from the pinhole lens. What happens? Place the screen at an angle to the light source. Measure the size of the image and compare with the actual size of an object in your picture. Use a plane mirror to reflect part of the light down on a tabletop. Is the image still inverted? Did the mirror reverse the image?
Place a simple convex lens against the pinhole. Enlarge the aperture so you have a camera with a lens.
The idea can be used with a large carton as your Camera Obscura with room for one or two observers. A smaller carton can also be used with only the head of the observer inside the box.
Warning: In your Camera Obscura, you can observe without being observed, so be discreet.