This paper describes the evaluation of an Intelligent Tutoring System (ITS) designed within the framework of the multidisciplinary AMBRE project. The aim of this ITS is to teach abstract knowledge based on problem classes thanks to the Case-Based Reasoning paradigm. We present here AMBRE-AWP, an ITS we designed following this principle for additive word problems domain and we describe how we evaluated it. We conducted first a pre-experiment with five users. Then we conducted an experiment in classroom with 76 eight-year-old pupils using comparative methods. We present the quantitative results and we discuss them using results of qualitative analysis.