Plummeting tax revenue has left schools across the country on the ropes, but in few places has the situation been quite so dire as in Oregon -- which has no sales tax, severe limits on property taxes, an automatic rebate on tax collections during boom years and a corporate tax structure that allows companies with multimillion-dollar profits to pay as little as $10 a year. Before the Legislature passed an emergency $51-million appropriation last week, the flood of red ink had threatened to close two-thirds of Oregon's 197 school districts before their scheduled last day. The prospect of early closures is particularly alarming in Oregon, where school calendars already are three weeks shorter than the national average.