"Harriet Harman has choice as a parent over which school is in the best interest of her son," said the borough's chair of education Anne Worsley. "But I regret that there is no Southwark school which she thought could provide a suitable education for him."
The fact that 11-year-old Joe will leapfrog several local comprehensives to get to St Olave's is a reminder that standards in the inner London borough remain among the lowest in the country.
Last year Southwark rose five places in the national examination league tables, with 22 per cent of pupils gaining five or more A-C grades at GCSE compared with 18 per cent the year before. But the borough was still fifth from bottom.
At Kingsdale comprehensive, the nearest school to the Harman household, only 6 per cent of pupils gained the top grades, while at William Penn, also close to the Harmans', the figure is 11 per cent.
The best performer among local authority comprehensives is Warwick Park, which scores 25 per cent. The grant-maintained schools in the borough do better, with the Roman Catholic St Thomas Apostle scoring 41 per cent.
But in Bromley, the school average leaps to 47 per cent, and at selective St Olave's, a half-hour train ride for 11-year-old Joe, 99 per cent gain the top grades. He could have followed his brother across the Thames to The Oratory in Fulham where he would join Tony Blair's son. The boys' grant-maintained comprehensive scored 62 per cent top grades last year.
Like many authorities, officers at Southwark admit that, however much they dislike such figures being bandied about, the league tables did help jolt them into action.
Two years ago they appointed a commission, led by former Inner London Education Authority chief Sir Peter Newsam, to raise standards.
As a result of its recommendations, the authority launched an action plan including special study skills and examination technique courses, revision classes, targeting of pupils with borderline grades, homework clubs and an ambitious scheme to strengthen links between teachers, pupils and parents.
Some results were dramatic. St Saviour's and St Olave's Church of England comprehensive for girls more than doubled its share of top GCSE grades from 17 to 42 per cent.
But Anne Worsley says the authority has its hands tied by its lack of control over school admissions. There are seven secondary schools run by the borough but there are two voluntary-aided Church of England schools and four grant-maintained Roman Catholic schools, over which it has no control.
However, Anne Worsley says that Joe is only one of many pupils who go elsewhere for their education, although a larger number travel into Southwark from outside.