Skagerak Gymnas was founded in 1991 by a group of enthusiastic individuals and companies from Sandefjord led by Elisabeth Norr. They believed there was a need to offer a non-selective alternative to the Norwegian state education system. The school established itself quickly in the revamped shipbuilding premises on Framnesveien 7 at Framnes.

The founders were committed to making the school a centre of educational excellence. When the school introduced the IB Diploma Programmeme (DP) it phased out the second and third years of the Norwegian national curriculum and changed its name to Skagerak International School. By October 1992 it was an authorised IB World School offering the DP.

IIn the meantime, Sandefjord Vekst (an association consisting of Sandefjord kommune and the local business community) had approached Elisabeth Norr in January 1998 to ask her to set up an international primary and middle school.

Considering the uncertainty of the market for such a school in a community the size of Sandefjord, it was determined that a new entity should be created. Thus Skagerak Primary and Middle School was founded in 1999 and opened its doors on Kjellbergveien 2 at Virik for the first students in August 2000.

The school grew very quickly and in August 2002 the middle school moved to the same location as the high school at Framnes, while the kindergarten and primary school remained at the Virik location until December 2011.

The leadership of the Primary and Middle school had the same high quality ambitions of the High school, and in June 2006 the primary school received its IB authorisation to offer the Primary Years Programmeme (PYP) while the middle school received its IB authorisation to offer the Middle Years Programmeme (MYP) in February 2008.

Skagerak International School is the only school organization in Norway to offer the full IB continuum, extending from kindergarten to year 13.

In 2008 the boards of Skagerak International School and Skagerak Primary and Middle School formed a committee to look into the possibilities of an even closer cooperation between the schools.