In 2000, ACST was granted authorization to offer the International Baccalaureate Diploma Programme (IBDP) in grades 11 and 12, becoming the only school in Tunisia authorized to do so. In order to meet IB requirements, a major building expansion program was undertaken, adding eleven Elementary and Secondary classrooms, two Secondary School Science labs, a Computer lab, a Library/Media Center, and a cafeteria/kitchen. In 2003, and due to a spike in the school’s enrollment, expanding to more than 500 students, another extension took place. This expansion, including four new modular structures, along with re-assignment of other spaces, led to a total of 14 new classrooms, new offices, and an additional Science lab. By August 2004 and with the African Development Bank establishing its headquarters in Tunis, ACST completed new High School and Middle School buildings (adding 12 new classrooms) and doubled the size of the cafeteria. By August of 2005, the original buildings had either undergone complete remodeling or were torn down. The only portables remaining were the Arabic/Tunisian Studies and Elementary ESL classrooms. A parking garage was constructed on campus in 2007. The extension work carried out on the school premises helped accommodate the increased enrollment, growing to almost 700 students.
In the fall of 2002, the new US Embassy facilities opened across the La Marsa Highway from ACST. In May of 2005, the Embassy recreational complex opened, which included a swimming pool, shared with the school to meet its physical education and competitive swimming needs.