PITY the backstage staff at Sha Tin Town Hall next month when 700 awe-inspiring young dancers take part in the biggest ballet bonanza in town. The dressing rooms will be overflowing, costume racks will block the corridors, and the make-up will be dabbed on by a production line of helpers. ''But the show itself will be quite spectacular,'' promised artistic director Jean Wong. The set of classical ballets, entitled Stars of Tomorrow and featuring a full version of Hansel and Gretel, will be the piece de resistance of a month-long international summer school organised for teenage ballet dancers. Ms Wong's regular pupils will join the visiting dancers to perform the show between August 14 and 17, with the directors working overtime to make sure the result is harmony and not havoc. ''It will be like a Cecil B. de Mille affair - hundreds of people in fabulous costumes performing at one time. It is quite a challenge,'' Ms Wong said.