What could be a high school experiment that involves changing a condition to get the highest yield of a certain product?
You could try the so-called Job's experiment. You prepare in advance 100 mL of two solutions of reactants which are supposed to have the same molar concentration. For example : CaCl2 0.1 M and Na2CO3 0.1 M. You take 9 test tubes in the same rack. Write 1 on the first, 2 on the second tube, etc. up to 9 on the last tube.

Now, with a plastic pipette, you put 1 mL of the CaCl2 solution in test tube 1, 2 mL CaCl2 in test tube 2, 3 mL of CaCl2 in test tube 3, etc. up to 10 mL in the tube 10. Then, you take another plastic pipette, and transfer 9 mL of NaCO3 solution in the tube 1, 8 mL of Na2CO3 in test tube 2, etc. up to 1 mL in tube 9. You mix thoroughly all tubes. You must see a white precipitate that settles in the bottom of  all tubes after a while. If you measure the height of the deposit, you will find that the highest amount is in the tube 5, if the stoichiometry is 1 + 1. If the mot important deposit is in another tube, it means that the stoichiometry is 1:2, oe 2:3, or any other ratio.

You may change and use any other combination of compounds. You will find that in the mixture CuSO4 plus NaOH, the stoichiometry is not 1:2, as you might suspect, as the precipitate is not Cu(OH)2, but 3Cu(OH)2·CuSO4.