Education Secretary William J. Bennett, citing multimillion-dollar mistakes in his agency's administration of student aid programs, formed a task force to seek ways to cut the waste. Bennett said that the Education Department has cracked down on defaulted student loans and recouped more than $260 million. But he said that audits of the $2.4-billion Pell Grant program indicate that there are mistakes--including overpayments and underpayments--in nearly one-fourth of the grants. "Such a high error rate . . . is absolutely unacceptable," Bennett said.