Experiment to demonstrate Dalton's Law of Multiple Proportions
Because of toxicity, I'd avoid middle or high school student experimentation with $\ce{CO}$.
You might consider $\ce{H2O}$ and $\ce{H2O2}$, if you stick to 6% (20 vol) or weaker. There is the complication of working with a dilute solution, though, in getting precise measurements on proportions.
Another possibility is demonstrating the various oxides of iron. However, there is another issue: you can produce fairly pure pure red iron(III) oxide, $\ce{Fe2O3}$, by the electrolysis of sodium bicarbonate, $\ce{NaHCO3}$ with an iron anode, but rather than pure black iron(II) oxide, $\ce{FeO}$, thermal decomposition of iron(II) oxalate in an inert atmosphere, makes a mix, wustite (a nonstoichiometric compound) that is primarily $\ce{FeO}$.
Chromium compounds, e.g. oxides, of varying proportions might make an interesting demonstration because of obvious color changes, but the toxicity of chromium compounds precludes use by students, and even safe disposal of the demo material may be problematic.
BTW, an interesting effect is observed in podzol soils, as found in pine forests. If there is iron in the soil, the reducing nature of freshly-exposed soil makes it look greenish; but after exposure to air, the iron is oxidized to a reddish color (I've observed the change in less than 15 minutes.).