Previous research has shown that teachers use an indistinct vocabulary, employ few concepts, and expose an embryonic professional language when talking about group work assessment, thus indicating a lack of a professional language. Building on Granström´s three different modes of language use everyday, pseudo-meta- and meta-language, the purpose of this article was to examine the teachers use of languages when talking about group work assessment. Specifically, if and how teachers use of modes of languages are influenced by them partaking in 1) a study about assessment in group work and 2) in an intervention in form of a short educational session. Data were gathered from interviews with eight teachers working in years five and eight in five Swedish compulsory schools and analysed both qualitatively and quantitatively. The results revealed that all of the teachers use Granstöms mode of languages to a varying degree when talking about assessment in cooperative situations. A core finding was that intervention in the form of a short education influenced the teachers way of talking in a positive way. By participating in the intervention, the teachers developed and expanded their mode of language, thereby promoting the use of a common professional language about group work assessment.