Theodore R. Sizer, dean of the School of Education, will become Headmaster of Phillips Academy (Andover) next fall. Last September, Sizer announced that he will resign from the School of Education this June.

In accepting his appointment as Andover's 12th headmaster in the school's 194-year history, Sizer, who is 40 years old, said yesterday, "I have been eager to return to teaching and secondary education, particularly given the stresses and changes afoot within the schools."

Under Sizer's eight-year tenure as dean, the School of Education tripled its budget, constructed two new buildings, and attracted renowned scholars to the faculty, despite recent drastic cuts in federal funding and the lack of an affluent alumni body to support the school. Sizer also radically revised the curriculum of the School last year.

Sizer's reputation as an innovator will be tested at Andover, which, like the other traditional New England boarding schools, is suffering from declining applications and dwindling financial resources. "With the likelihood of increased state funding, high schools will become more and more alike," Sizer said yesterday. "That puts responsibility on private schools to come up with new alternatives."

Andover is an all-boys school, although it does conduct a limited "co-ordinated" education program with neighboring Abbott Academy, an all-girls school, Sizer said yesterday that he is "definitely in favor of co-education at Andover."

Sizer is not new to secondary boarding schools. He attended Pomfret School before graduating from Yale in 1953, and taught at Melbourne Church of England grammar school in Australia in 1959.

Sizer's degrees include a Master of Arts in Teaching from Harvard in 1957 and a Ph.D. in American History and Education from Harvard in 1961. He stayed at Harvard after completing his Ph.D., becoming an assistant professor of Education and director of the Harvard Master of Arts in Teaching Program. In 1964, he became dean of the Faculty of Education, succeeding Francis Keppel, who had resigned to become U.S. Commissioner of Education.