If middle schoolers are grade 7s and 8s ( I teach in Canada), then I would suggest the following inquiry activity; Bring a plant from home: a geranium works well, and expose the plant to intense light for 24 hours (under a fluorescent light), but have one of the leaves covered up with a little strip of aluminum foil (only part of the leaf). The strip has to be maintained tight with a clip, so that no light will be able to enter. After 24 hours, if you have the proper materials, look for starch in the leaves, actually only in that leaf you had covered up (part of it) - You will have to cut out that leaf. First, put the leaf for a few seconds in boiling water, then put it in hot alcohol (this is dangerous - keep the students away), wait for the green color to be gone, then take out the leaf from the beaker, wash it with water, and lay it on a petri dish: you will now be able to put iodine over the leaf. The area not exposed to light should be more pale, because no photosynthesis occurred in that area, therefore less starch was produced. (Sometimes, you need to wash the leaf before you can see this). An easy discussion can follow: how? Why?, etc. Good luck! Let me know if it helps.
There are also simple things you can do: have the students grow plants from seeds, and while they grow, discuss with them what is happening (does the plant grow in the dark, when does it turn green, etc,)