These figures are meaningless.
Why are they comparing infections in school staff against the general population? It is only meaningful if they are comparing it against the working age population.
Are school staff more likely to take a test than an average person? (Almost certainly). So does this data mean what they say it means?
And how did they calculate their estimates? They have linked to a spreadsheet they made themselves. It doesn't refer back to data sources and it includes formulae like "* 500" with no explanation about what it means.
If the analysis was done by an independent statistician, rather than by a trade union who has been trying to close schools every month since the pandemic began, I would be more inclined to believe it.
As it is, this does not seem at all credible.