Mother Seton Academy, an experimental Roman Catholic middle school opened by six religious orders in September in a vacant building of St. Patrick's Parish on Broadway, will expand this fall and relocate four blocks south to a larger church building in Fells Point.

The move to the former convent of St. Stanislaus Parish on South Ann Street was made possible by a grant of $75,000 from the Abell Foundation, Xaverian Brother Arthur Caliman, the academy's board chairman, said yesterday.

The free school, which opened at St. Patrick's with 20 boys and girls in the sixth grade and a staff of six, will add a seventh grade and double the enrollment to 40 students.

"This new space will allow us to have all our needs, including a cafeteria, located in one building and will provide room for us to grow into a full middle school program with sixth, seventh and eighth grades the following year," Brother Arthur said.

Original plans for an enrollment of 30 boys and 30 girls from poor "white, African-American, Hispanic and Native American families" in East Baltimore by fall 1995 are on track. The purpose of the academy's intensive, all-day curriculum is to rescue teen-agers "falling between the cracks" of public and parochial schools, Brother Arthur said.

Also announced was the promise of a substantial gift from donors in Washington. The academy's first class of 20 students will receive $75,000 from the H. V. Robertson Family Memorial Educational Foundation to assist with college or vocational training after graduation from the academy and high school.

"It is our hope that this promise of future support will motivate each of these pioneer students to achieve their fullest potential at both Mother Seton Academy and throughout their high school years," said Sister Jacqueline Marie Kotz, a member of the Franciscan Sisters of Philadelphia and the academy's co-principal. The other co-principal is Sister Ellen Smith.

In addition to the Philadelphia-based Franciscan Sisters order and the Xaverian Brothers, the groups making up the partnership that planned and opened Mother Seton Academy are the School Sisters of Notre Dame, the Daughters of Charity, the Immaculate Heart of Mary Sisters of Scranton, Pa., and the Franciscan Sisters of Baltimore.

Brother Arthur said a seventh religious order, the Conventual Franciscan Friars, helped with the move to South Ann Street.

The academy's program extends from early morning to evening and includes recreation, work periods, supervised study and individual tutoring in addition to regular classes.

Free breakfasts, lunches and suppers are provided with federal nutrition assistance administered by the Catholic Archdiocese of Baltimore.

A similarly intensive middle school program in Baltimore is provided by the Jesuit Fathers at St. Ignatius Loyola Academy, which opened last fall in the old Loyola College building in the 700 block of N. Calvert St.