SOCIAL service agencies are in danger of failing to recruit enough voluntary workers to help during flag days because secondary school students are shunning the activity. Social workers are worried the problem would become more acute on weekday drives because on these days students are not free to help. This year, the government has approved four organisations to sell flags on weekday mornings due to the large number of applications. Figures showed the government received more than 100 applications to help with flag days during 1991 and 1992. But as only 44 flag days were assigned, suggestions have been forwarded to increase the number of events or organisations will be turned down. While welcoming the government's move to increase the flag day count, Mr Edward Leung Wai-kuen, executive director of the Hongkong Playground Association, questioned if there would be enough helpers. ''Volunteer recruitment is becoming difficult as students, who are busy studying or joining extra-curricular activities, are unwilling to be involved in voluntary work. ''Flag selling is no longer interesting to them,'' he said. Mr Leung said students were their main source of manpower during the association's annual fund-raising flag day. Among the 3,500 volunteer workers needed each time, almost 2,000 or more than 57 per cent were students recruited from schools, with the remainder coming from the association, Junior Police Call and other voluntary groups. However, the response from schools in recent years had not been encouraging, said Mr Leung. Last year, the association sent 349 invitation letters to most of the territory's secondary schools six months before the event. But only 82 of the schools promised to help. He said student recruitment would be harder if the fund-raiser was held on a weekday. ''I don't think many schools will permit students to take a half day off to help with flag selling, as they fear their academic performance will be affected.'' In a letter recently published in Young Post , a ''disappointed'' secondary school student wrote: ''One public organisation recently asked us to sell flags, however, the school authority just forced us to join the activity. Although we were keen to join we were put off by this high-handed attitude.'' However, Mr Wong Yiu-so, principal of CCC Kwei Wah Shan College, said some schools also had a tough time. ''We want to help but sometimes there are too many for our students to handle. Last year we received almost 50 invitation letters to sell flags, but at most we can only help with five.'' Mr Wong was also concerned about student safety and behaviour while on the streets selling flags. He recalled how a few years ago three students were caught by the police for stealing money from the collection bags after their fund-raising duty. ''Since then, we help little with flag selling in case our students should be given another chance to commit a crime,'' he said. ''Since students are not on the school premises, no teachers are there to supervise them.'' On flag days voluntary workers are given a collection box or bag, a map, and instructions on where and how to sell.