The objective of this study was to apply a Rhetorical Structure Theory (RST)-based diagnostic tool to assess the text organizing ability of elementary and middle school students in order to examine how they performed their tasks in each grade. To this end, we developed a diagnostic tool constituted by some tasks and conducted statistical analyses on the scores of 478 elementary and middle school learners for each task and group. Our findings are specifically summarized as follows. First, the differences between the task scores of elementary and middle school students were statistically significant in all grades. The two tasks for which the scores were markedly low presented greater numbers of “relations” and “types of relations” than the other tasks. We also observed that the lack of nuclei contributed to the difficulty of these tasks. Second, the analysis of inter-group differences using the overall average scores of students of all genders and grades showed significant inter-group differences across grades. The analysis of the text organizing ability through the differentiation between the ability to distinguish and arrange text indicated the presence of significant inter-group differences across grades. However, these differences were not found across genders. Third, a detailed analysis of the scores of elementary and middle school students by grade, gender, ability to distinguish text, and ability to arrange text indicated statistically significant differences in scores across grades. However, no interaction effects were found between grade and gender. Fourth, we found statistically significant differences across grades in terms of ability to distinguish and ability to arrange text. This implies that it would be possible to give feedback on the assessments on each sub-area of text organizing ability depending on the characteristics of each student group. Despite its findings, this study is limited because it assesses the text organizing ability of students based solely on the arrangements of given sentences. However, this study is significant because, through efforts to validate the diagnostic tool, it has paved the way for a relatively simple, expedient, and valid method of assessing the text organizing ability of a large number of students.