I don't think Wisconsin is a right-to-work state. So, teachers are required to join the union as a condition of employment.
If dues aren't deducted automatically, and they don't pay them voluntarily, it seems to me they'd be tossed from union membership, which would
mean forfeiting their jobs, unless closed shops only apply to private enterprise.
Or maybe "voluntary" dues payment is one of those union deals where it's voluntary, but there's always a risk your house may accidentally catch fire.