This study aimed to evaluate textual coherence in explanatory texts written by third-year secondary school students. To do so, the participants, 47 French-speaking third-year secondary school students from a school located in the Montreal area, were asked to write an explanatory text on the therapeutic effects of laughter. To assess the coherence of these texts, appropriate markers and errors related to themes, connectors, and arguments were compiled. Our results indicate that the students tended to use a constant theme progression with a high rate of errors in thematic recurrence, omission of essential connectors, use of inappropriate connectors, and illogical arguments. The interpretation of these results allows us to identify the strengths and needs related to textual coherence with a dual aim: to inform future intervention research on coherence and to support practitioners in their efforts to help writers in the age group of our participants improve textual coherence in their texts.