GRADUATES of Holy Trinity College will soon discover that the most important thing they have acquired in secondary school is not textbook knowledge or special skills. ''The most vital thing is that you have learned how to be a good, virtuous and talented citizen,'' said the Right Reverend John Tong Hon, Vicar General of the Diocese of Hongkong. Speaking at the school's 25th speech day recently in Sha Tin Town Hall, the guest-of-honour said the graduates not only acquired knowledge and skills in school, but also morals and ethics which would help them to understand and live life better. ''The school gives you a chance to develop a stronger body and a more cohesive mind. It also teaches perseverance and caring,'' he said. ''It is now the time to 'give a thought to the fountain when drinking the water'. You should follow your school's guidance and seek the source of life.'' He also told the graduates that if they thought secondary school was tough, they had better be prepared because they would run into more difficulties in tertiary institutions or in the working world. If the graduates had problems getting along with schoolmates, they would have even more problems in the outside world, he said. In his annual report, assistant principal Mr To Yiu-kwan said the school had 1,172 students in 31 classes in the 1992-93 academic year. Mr To said the students had a a 98.3 per cent pass rate in last year's HKCEE, and some 50 per cent of the candidates scored distinctions and credits. Meanwhile, 81 A-level candidates secured a place in local universities or tertiary institutions. The assistant principal attributed the success to the high quality and dedication of teachers and the hard work of students. Stressing that the school placed equal emphasis on academic performance and extra-curricular activities, Mr To said: ''We have more than 30 academic, service, religious, sports and interest clubs and societies for our students.'' The Reverend John Tong presented certificates and prizes to the graduates and outstanding students. Fifth-former Peggy Man Kit-yee won subject prizes for Chinese Language, Physics, Chemistry, Biology and Computer Studies. On behalf of the graduates, seventh-former Edith Lui Wing-yin expressed her gratitude to the principal, Sister Rose Mary, and all the teachers who devoted their time and energy to the students.