Argument skills have received a lot of attention in recent years. However, few studies report on the argument skills of elementary students, resulting in a lack of knowledge base for learning support. Krajcik & McNeill (2009) propose a hypothetical learning progression for scientific explanations which might grow across grades 1-12. This study focuses on the first variation of that progression. In the simplest variation students should be able to provide a claim and support that claim with evidence. This study aims to 1) give in-class lessons to elementary students designed to help them acquire argument skills and 2) evaluate the effectiveness of the lesson design principles from the perspective of the skill of formulating arguments. From earlier studies, this study extracted three design principles: 1) learning thoroughly about the basic knowledge that concerns the problem domain, 2) collecting multiple pieces of data, sharing them in the classroom, and organizing them into several groups, and 3) making students aware that they are using evidence. In line with these design principles, we gave classes concerning the issue of coexistence of humans and wild animals for third graders. Analyses of results of surveys conducted during the classes revealed a significant increase in the number of students who used evidence to formulate their arguments, indicating successful acquisition of argument skills.