This paper is about how serious games based on MUVEs in formal education can foster collaboration. More specifically, it is about a large case-study with four different programs which took place from 2002 to 2009 and involved more than 9,000 students, aged between 12 and 18, from various nations (18 European countries, Israel and the USA). These programs proved highly effective into fostering a number of transversal skills, among which collaboration (both remote and in presence), stood out as prominent. The paper will introduce the four programs, the way they were designed to foster collaboration and the data on their impact.