HARD work was rewarded when Sha Tin Government Secondary School girls won the top prize in a pi-pa ensemble contest at the 46th Hong Kong School Music Festival on Tuesday. The contest, which took place at Hong Kong Schools Music and Speech Association, required the five participating schools to perform a piece of music of their own choice. The five-member team played ''The Song of Guerilla Forces'' and said their victory was a big surprise. ''We never thought of winning and we came here just to gain more experience,'' said jubilant team leader Lung Man-lee. The group spent about three months practising for the competition which involved a lot of hard work. The team also attributed their success to the art of being as ''relaxed and natural on the stage as possible''. ''When we first got on stage, we were really scared but later we became more relaxed and just let our feelings and skills speak for themselves,'' the group members said. According to adjudicator Lam Fung, the group displayed excellent co-ordination in handling different parts of the piece. ''They performed not only in a harmonic way but also co-operated well with each other. This is quite difficult when you're playing music which is divided in different parts,'' said the veteran pi-pa player. Carmel Pak U Secondary School, which had the largest group of seven, won second place. ''We did not have much confidence since we did not have enough time to practise,'' said leader Wong Wai-hong. ''The prize was an encouragement to us, but we considered this more as an opportunity to learn than a competition,'' said member So Mei-fong. The team took less than a month to prepare. Considering the limited preparation time, Wai-hong said the success was the result of their receptive minds and an open attitude towards suggestions from others. Both groups expressed their desire to join the contest again next year, with their abilities proved and confidence boosted in this year's competition. STFA Leung Kau Kui College, who were taking part in the contest for the second time, won third place. RESULTS 1. Sha Tin Government Secondary School, 88 points; 2. Carmel Pak U Secondary School, 84 pt; and 3. STFA Leung Kau Kui College, 82 pt.