Having come from the tourists' beach, Jerry is clearly an outsider and foreigner, as well as a younger boy to the older, dark boys who swim from the rocks.
There are, also, innuendos of the racial divide of apartheid, with which Lessing was familiar, having grown up in Rhodesia (Zimbabwe). Because of this situation, the boys are probably surprised that Jerry desires interaction with them, and they depart quickly lest some adult appear and accuse them of being "out of place," etc. Perhaps, though, the primary assessment of the older boys is one of disdain for Jerry's juvenile behavior as he shouts what French he knows to the boys and splashes around acting silly.
That the boys perceive Jerry as juvenile is apparent when Jerry calls out to them and they "looked at him idly and turned their eyes back toward the water." Later when he shouts "Look at me! Look!" they "looked at him gravely, frowning" with a similar look to that of his mother when Jerry would fail and she gave him "this grave, embarrassed inspection."
After the boys swim through the tunnel, they gather their things and run off in order to "get away from him." When they rush off, Jerry, left behind, cries. It is then that he decides that he must swim through the tunnel and prove himself.
The part which best describes Jerry's place among the boys is the following:
They shouted cheerful greetings at him; and then, as he preserved his nervous, uncomprehending smile, they understood that he was a foreigner strayed from his own beach, and they proceeded to forget him.
One of the boys had smiled and waved at him which encouraged Jerry to join them. It is clear, initially, that they were keen to welcome him because their greetings were 'cheerful.' This demeanour changed, however, when he could not understand what they were saying but he continued smiling which, to them, must have seemed quite foolish. These actions convinced them that he was an outsider who had wandered off from where he was supposed to be.
'...they proceeded to forget him' powerfully indicates how dismissive these boys were of him. He was not their company at all. Even in their midst, he was not someone to be remembered. He might as well have been part of the landscape. Jerry had not made any kind of impact that would make his presence worthwhile. The fact that he was a foreigner is what probably made them reject his previous appeal for friendship.
Jerry's place amongst these boys is, therefore, that he has no place at all. He had been acknowledged and then dismissed from their thoughts almost immediately. They do not see him as a friend or someone they wish to keep company with. Jerry's later actions do not impress them at all and once they have achieved what they came to do, they left to get away from him.
Their brutal rejection was a painful experience for Jerry and drove him to tears. His young ego had been terribly bruised. However, it also made him determined to prove, to himself, that he had been worthy of at least some of their attention and respect since he could do exactly what they did, even if he almost lost his life in the process. With this purpose served, Jerry found it unnecessary to go to the bay again.