The Web has allowed the publication of many pedagogical resources. However, due to their important number, it is difficult to manage and exploit them efficiently. One way to solve this issue is to describe the resources with informations taken out from metadata schemas. This solution is not sufficient because schemas are not really shared formalisms and may contain semantic ambiguity. For preventing such ambiguities, we chosed to build an ontology whose objective is to better define the elements contained in description schemas. In this paper, we present the ontology building process used for indexing pedagogical resources. We precisely define our concept of semantic indexing. We present also the tool we have developed for indexing and retrieving pedagogical resources in a local knowledge base.