Pupils and teachers at St Saviour's and St Olave's School near Bricklayers Arms last week attended a traditional 'topping out' ceremony to mark the structural completion of an extension to the school's buildings.
Headteacher Dr Irene Bishop CBE was asked to lay some concrete before proposing a toast to the workforce for a 'job well done'.
Construction began at the girls' secondary school in July 2011 and the whole project is due to be completed in October this year.
The extension has been designed by AOC Architecture.
"Balfour Beatty as part of 4 Futures, the Southwark Education Partnership, are delighted to be delivering this important addition to St Saviour's and St Olave's School," says Kevin Pluckrose, Balfour Beatty operations director.
"The new building will provide additional science, ICT, music and sixth form facilities.
"The structure of the new building is now complete and fitting out is in progress for completion at the end of October."
Cllr Dora Dixon-Fyle, Southwark's cabinet member for children's services, said: "I am very happy to see another Southwark school develop and enhance its learning facilities for the benefit of its students and I'm looking forward to seeing many more schools open new classrooms and learning spaces in the future."
The school is one of three associated with Southwark Cathedral and the Dean of Southwark, Very Revd Andrew Nunn, was among the guests at last week's ceremony.