AN influx of mainland children means there will be a shortage of more than 500 Form One places next year. As an indirect result, 5,000 youngsters will have to attend schools a long way from where they live. A paper to be discussed by the Board of Education on Monday says a preliminary examination of supply and demand this year indicates the situation is tight because of an unexpectedly large influx of immigrants. The paper shows an overall shortage of 503 places. Analysis of individual districts shows more than 5,000 students will be forced to attend schools outside their residential areas. But Assistant Director of Education, Hsu Show-hoo, said there would not be any major shortage, because 400 children were likely to drop out by the end of this school year because of emigration and other reasons. The problem would also partly be solved by schools adding two ''discretionary'' places per class, he said. The shortfall was unexpected, he said, because the original estimate of 85,000 Form One places would mean a surplus of 700 places this September. But with the Government's decision to increase the quota of one-way permits for mainland children by 15 every day, a shortfall of 503 places was projected. The influx was also partly blamed for the need for 5,486 students to study in schools outside their residential districts. Mr Hsu said mainland children tended to live in Hong Kong East, Kowloon City, Tsuen Wan, and some New Territories districts along the railway line. The paper says: ''Some of the pupils in [Central and Western, Hong Kong East and South] will be allocated to Wan Chai and 360 will be allocated to Kowloon schools situated along Mass Transit Railway and Cross-Harbour Tunnel bus routes.'' In the New Territories, 547 students will probably be sent from Tsuen Wan to Kwai Chung, 735 from Tuen Mun to Yuen Long, and 243 from Sai Kung to Kwun Tong. More than 1,000 students in the North, Tai Po and Sha Tin will have to study in Kowloon schools. Legislator Tik Chi-yuen said the Education Department needed to review school building plans in light of the changing population pattern. Mr Hsu said arrangements had been made to advance the opening of two schools in Yuen Long and Tai Po by one year to 1996 while temporary measures included making use of vacant primary school buildings in Tai Po. In the long run, more new secondary schools will be built to cope with the additional 5,475 mainland children every year, of whom up to 70 per cent are at school age. Under the secondary school places allocation system, most Primary Six pupils are allocated to secondary schools located within their residential districts while the local shortfall is to be met by the surplus in adjacent districts.