THIRTY secondary school students took off for an exchange camp in Beijing and Nei Meng Gu recently.. Called ''Hong Kong-Mainland Youth Summer Camp'', the programme was organised by the Hong Kong Federation of Youth Groups and the All China Youth Federation. The camp gives the 30 participants the chance to be involved in a series of activities, together with a group of 60 secondary school students from China. A federation spokesman said the event aimed to encourage mutual understanding and friendship between young people. ''The opportunity to take part in visits, seminars, meetings and to share accommodation allows the Hong Kong delegates to understand more about the lifestyle of youngsters in Beijing and Nei Meng Gu. ''At the same time, they will be able to exchange ideas on the common problems of youth.'' The spokesman added that in order to make the most of their tour, the Hong Kong delegates had undergone a few months of special training. They learned Putonghua and formed groups to gather relevant information. Before returning to Hong Kong at the end of the month, the delegates will attend a ''Seminar on Youth Issues'' and a ''Sharing Session between Beijing and Hong Kong Students''.