Yellow highlighters' coloring is called 'Solvent Green 7' and has a peak at about 494nm.
You can buy filters for lenses that will knock out certain bandwidths of light and keep the rest coming into the camera. These are called notch filters, as they take a notch out of the light.
There just so happens to be a notch filter that should know out the yellow highlighter color. Meaning that the light reflected from a yellow highlighter will not enter into the camera.
https://www.thorlabs.com/thorproduct.cfm?partnumber=NF488-15
They are not cheap though, and as such, I've not tried this at home to see how well it works.
Other solutions may exist at to the ideas that these people may have about how your webcam is working. I'd experiment with other lenses, polarized filters, exposure times, etc.
That said, studying is a lot easier than trying to jerry-rig your webcams.