THE Commissioner of Police, Mr Li Kwan-ha, is inviting secondary school-leavers to join the Royal Hong Kong Police Force for a career. A direct mail package, which included a letter of invitation by the commissioner, was sent to secondary school graduates recently. A spokesman for the police said: ''This is the third attempt that secondary school-leavers are being approached since 1991.'' The force managed to recruit 700 school-leavers in this first year while last year, 1,217 school-leavers were recruited. ''The commissioner is particularly keen about school-leavers because they are academically qualified for police work, especially in the light of a present policy to upgrade the quality of police officers.'' Alongside the direct mail package, the recruiting campaign will commence in full force on all fronts in the coming months, featuring a brand new recruitment commercial on television and in the cinema, radio advertising, newspapers and magazine advertising, career exhibitions and posters display at MTR stations and government public poster sites throughout the territory. The commercial, again an action-packed true-life documentation of the police at work, will give prospective recruits a better insight into the police career. It will show officers dealing with armed robbers, violent criminals; casualties in traffic accidents; common folk in domestic disputes and victims of natural disasters. ''The police are always there to serve and protect, but this has been taken much for granted by the general public,'' the spokesman said. Thanks to the campaign, the once very acute manpower situation has been greatly alleviated.