Almost half of the teachers at St. Patrick's Episcopal Day School have decided to join the school's ousted principal to form a new Washington Episcopal Day School next fall.

Isabelle Schuessler, who was fired last month after 20 years as director of St. Patrick's, has been named head of the new school. Its board includes former members of the St. Patrick's trustees who resigned after Schuessler was fired by the Rev. S. James Steen, the pastor, in a dispute over dividing authority and funds.

St. Patrick's is in a large modern building, shared by church and school, at 4700 Whitehaven Pkwy. NW. The church moved into a new $3.5 million wing a year ago.

Robert E. Freer Jr., chairman of the Episcopal Day School's trustees, said the new school plans to operate next fall with nursery classes up to fifth grade in rented space at Pilgrim Lutheran Church, 5500 Massachusetts Ave., Bethesda. He said it expects about 120 students, about half from St. Patrick's, and has already hired 13 of St. Patrick's 30 classroom teachers, along with one assistant teacher, plus the school's secretary and psychologist.

Overall, St. Patrick's may lose about 100 of its 375 students because of the bitter dispute, several parents said. But school officials have expressed confidence that the departing teachers and students will be replaced and quality will be maintained.

"The people who want to leave are leaving, and the people who want to stay and build are staying," said Victoria P. Sant, chairwoman of the St. Patrick's trustees. "It's sad that things happened this way. But I think there is room for both schools. There's a shortage of good private schools around here. The competition to get in is so fierce."

St. Patrick's, which began as a nursery school, started adding elementary grades in 1966, graduating its first sixth graders in 1974. Among its recent graduates are the children of Joe Albritton, the president of Riggs National Bank, and Sen. John Danforth (R-Mo.). Its current group of parents includes Sen. Ted Stevens (R-Alaska) and journalists George Will and Bob Woodward.

Steen, who has been pastor of St. Patrick's for six years, has declined to be interviewed about the firing of Schuessler. But at a meeting the day after her removal he said he could not function effectively as rector with Schuessler as principal. He has received strong support from Episcopal Bishop John T. Walker, head of the Washington diocese, who said that angry parents at the meeting were behaving like a "lynch mob."

Freer said he hoped the new school will develop its own ties to the diocese but has not yet approached Walker about it. Its tuition next year, he said, will be $5,000, just $25 below St. Patrick's.