Mountain View, Calif.--Public school children of St. Joseph's Parish here pose outside the 35-foot trailer-school in which they attend released-time religious instruction classes. The Rev. James B. Doyle of St. Joseph's, faced with a mushrooming congregation of San Francisco commuters, solved his scattered flock's educational problem by getting this 2 1/2 ton trailer for $2,800. The vehicle, which seats 55 pupils, pulls up to each public school before the youngsters are released. Father Doyle said that 900 public school children are under released-time instruction. St. Joseph's Parish is in the heart of a country area being rapidly developed with homes for persons working in San Francisco, 30 miles to the North.