With the termination of the uncertified teachers and the expected retirement of hundreds more, school system officials said they could have up to 750 vacancies this summer, which would be almost double the usual number. School human resources officials said they have 950 teacher candidates assembled through widely expanded recruitment efforts, including at job fairs in Philadelphia and Detroit. Still, they are working hard to replace 50 librarians, 50 math teachers and 100 special education teachers--high--demand jobs for which there is always a short supply. About 40 special education teachers were among those dismissed June 30, intensifying the shortage. 'Do we have enough in the pool to fill the math and special education vacancies? No,' said Nicole Wilds, the school system's director of recruitment services. 'We're interviewing 30 candidates a week to get them into the pool.' Although the 370 uncertified teachers who were fired represented a range of subject areas, those in the largest group--41--taught special education. Those in the second-largest--20-- taught in the sciences. About 15 of the teachers taught math. About 290 of the teachers had been hired since 2000. Most of the teachers, who had provisional (临时的) licenses, failed to complete the college coursework needed for full certification, officials said. As many as 1,400 teachers--about 25 percent of the teaching force in the city--were uncertified as of March 2005. In January, Superintendent Clifford B. Janey announced that he was setting a deadline of June 30 for those teachers to obtain their credentials (证书)--or face dismissal. The 370 who lost their jobs were part of a total of 820 who were still uncertified in June. School officials, saying they recognized it would be nearly impossible to fill more than 1,000 vacancies from the loss of uncertified and retiring teachers, decided to give 450 of the uncertified instructors an opportunity to remain with the school system for a year. What is the primary reason for the big shortage of teachers this summer?