More than 2,100 teaching positions were vacant in Pennsylvania as of October, according to newly released state data — and almost a quarter of those spots were filled not with full-time teachers, but with long-term substitutes. According to reports, the vacancies, reported by the state for the first time, are one indicator of the severity of Pennsylvania’s ongoing teacher shortage — but not the extent of it. Accounting for the number of teachers leaving the profession each year as well as those who are teaching with emergency permits, Pennsylvania schools need to hire at least 15,000 teachers — nearly three times the number the state certified last year, according to a new report.