Just don’t use accounts the school knows about…how hard is that? Don’t use the shitty equipment they give you because it’s loaded with spyware, haw hard is that.

From the article (emphasis mine):

The research team identified 14 companies actively marketing online surveillance services frequently beyond school-issued devices and outside of school premises, raising concerns about privacy, equity and oversight

Sounds like they’re using heavier tracking than simple device tracking. Probably lots of fingerprinting, which can be extremely hard to avoid; If any of your accounts even touch the same network as the school device, they likely track it.