Since the delivery she has taken the Vermox again to destroy the eggs and plans to do it again in 3 weeks. By some estimates, close to one-fifth of all children in the United States have pinworms at any given time.
Young children tend to be most effective at ingesting and spreading pinworm eggs.
Most likely, then, you're correct in guessing that your wife picked up these little buggers in her classroom. The length of time that you've had pinworms should not affect the treatment or make it more difficult to get rid of them. It's not as if the pinworms will erect an entire city in a year's time, versus a squatter settlement if given only a few weeks. The mebendazole is one of the most effective drugs out there for getting rid of pinworms, as well as other helminth relatives of the pinworm. To treat pinworms, you only need to take one 100 mg tablet at first.