Lowering class size by just a fraction of the degree sought by class-size reduction advocates would require a tremendous expansion of the Department of Education’s budget, Deputy Chancellor Christopher Cerf just testified at today’s Assembly hearing on mayoral control in the Bronx.

Recent DOE analysis concluded that a reduction in class of 10% — from an average of 25 to 22.5, for example — would cost $800 million a year in extra operating funds to pay for new teachers, Cerf said. Constructing the extra classrooms needed would be an additional tens of billions of dollars in capital funds, he said.

The city last year received $150 million from the state in funds earmarked to reduce average class sizes in a set of needy schools.