History of IMC
In 1997 the IMC program started in the UK see report by Roger Barlow in CERN courier 2014. The next step happened in 2005, the World Year of Physics, when the program was organised in many European countries by EPPOG (European Particle Physics Outreach Group, since 2010: IPPOG, International Particle Physics Outreach Group). International Masterclasses 2005 were the first European-wide Masterclass event. In 2006, U.S. institutes joined the program. Today, International Masterclasses are held worldwide in 60 countries. Pictures, agendas and schedules of past events can be viewed here.
A symposium and celebration of the founding of the International Particle Physics Outreach Group in 1997 (25th anniversary) can be seen under this link.