WITH the completion of five extra classrooms which would end its ''floating class system'', the 15th anniversary of St Teresa Secondary School was made especially meaningful. Because of the limited number of classrooms, the senior students at St Teresa have had no home-room and instead moved from room to room to attend lessons. However, with the campus extension project, which added one floor to the five-storey school tower, all students could now be settled, according to St Teresa's principal, Ms Hilda Kwan Wei-yin. The $1.6 million project was paid for by 10 years of school savings and two fund-raising walkathons held in March and November. While expressing gratitude to project sponsors - including past students and pupils' parents - Ms Kwan was disappointed about the failure of the Education Department to offer help. She said it took too long to apply for grants for a large-scale project from the department. Using a renovation project which had been awaiting approval for three years as an example, Ms Kwan said she had doubted ''the campus extension project would be approved by 1998''. In view of the urgent need to give students a better study environment, the principal said they decided not to wait for the department's aid and took up the project themselves. Ms Kwan said she hoped the department would abolish the floating class system, still being practised in other schools, as soon as possible. ''Students studying under the system suffer from the constant changing of classrooms. They easily lose their attention to studies and sense of belonging to the school,'' she said. She also said 40-minute lessons became shorter under the floating system because it took several minutes for the students to change their classrooms. Two fifth-formers, Peggy Lam Wai-lin and Wilphia Yiu Ka-po, told Young Post they were happy to end their ''nomadic experience''. Meanwhile, St Teresa held a three-day open day earlier this month to commemorate the school's 15th anniversary. Past students and pupils' parents visited student exhibitions with topics ranging from academic subjects to extra-curricular activities.