As you are looking for inventive ways to possibly measure Fe presence for a school presentation, may I suggest a crude (but likely interesting) visual aid in possibly determining relative iron concentration.
One idea is that Fe ions in the presence of acidic H2O2 will engage in radical production. The latter reaction in the presence of suitable light over time could result in a cyclic reaction system (for example, photo-Fenton). The latter is a conceivable path to magnify even small transition metals presence.
The latter radicals (especially, the powerful hydroxyl radical) can breakdown organic matter, including, for example, a strongly colored dye over time, resulting in eventual de-colorization. The relative periodic time evolution for the latter could be a manner to simply display/document apparent differences in even small relative iron (also copper, manganese,..) concentrations.
Here is a citation from the literature as an example, Effect of iron salt on the color removal of water containing the azo-dye reactive blue 69 using photo-assisted Fe(II)/H2O2 and Fe(III)/H2O2 systems.
So, spinach grown in iron-rich soil, reduced to ash added to acidic H2O2 (plus perhaps more acid added 1st to the spinach, further lowering pH) in the presence of a photo-sensitive dye subject to photolysis, should display more rapid de-colorization (as captured by periodic snapshots) than one grown in iron-poor soil.
You may experiment with various ways of numerically quantifying the tentative iron content, as deemed viable given limited resources (including funds which alternatively, could purchase an iron testing kit, available here, for example, at $99, or less expensive water testing strips here).
Note: I am assuming one does not have the time to actually grow the same variety of spinach. Otherwise, I would simply speculate that iron-richer spinach may possess different coloration (and perhaps other discernable quantifiable attributes) for the purpose of a simple relative classification as to iron content.
This would conceptually be based, relatedly to above, on naturally occurring photosynthesis. To quote from a supporting source:
Iron, one of the structural elements of organic components that play an essential role in photosynthesis and nitrogen assimilation of plants, is available at extremely low concentrations in large parts of the Southern Ocean's surface waters. We tested the hypothesis that photosynthesis is the primary target of iron stress in phytoplankton living in this specific environment, resulting in a reduced carbohydrate production.
On the suggested coloration argument relating to iron content, also a confirming reference, to quote:
Leafy vegetables featuring a deep, dark green color, such as spinach and collard greens, are usually high in iron.