Suppose that Kastoroff's bid was not just $10,000 lower than it should have been, but $50,000 lower than all the other bids. When the school board opens the bid, it realizes (or should realize) that Kastoroff must have made a mistake.
If the school district accepts the bid anyway, the contract is voidable.
Similarly, suppose the school district was at fault for the mistake. Suppose it required that bids be calculated using an online calculator supplied by the school district. The calculator contained a programming flaw that cause errors in certain cases. The school district is at fault for the mistake, and the contract is voidable.