Building on a historiography that is in full expansion, we are focusing our attention on the sociogenesis of "educational internationalism", by studying the way in which agents and organisations which claim to belong to this movement have executed their commitments and reconfigured them over the decades. After having studied the groups which work within the International Bureau of Education (IBE) -- which aims to build peace through science and education -- here we are examining the way in which its director, Jean Piaget, shaped the implements and the operating methods of the IBE, and represented it on different international stages. The well-endowed archives that were analysed have prompted us to suggest that he is revealed as a diplomat of "educational internationalism"; while the IBE became the first intergovernmental agency (1929), it started working with UNESCO from 1946 before it became fully integrated in 1969. In particular, this article shows how, within the intergovernmental context of the IBE, this learned man adapted the concepts of teamwork and self-government that he had theorised. We are interested to see which diplomatic tools Piaget put to use in order to tackle the unavoidable contradictions that he came up against. How was one to preserve a strict neutrality and scientific objectivity, conditions of an educational internationalism which claims to be universalist, in the arenas where it is governmental and institutional delegates in struggle that debate, moreover on topics such as school -- viewed as a space where national identities are fashioned?