Introduction: Writing is a tool for constructing and modifying knowledge. However, it is not easy to evaluate spontaneous writing tasks. In the present study, we selected and analysed a series of quality indicators to assess spontaneous or productive writing, examining the level of difficulty of the indicators and the relationship between the application of metacognitive strategies when writing and the quality of spontaneous writing products. Method: Participants consisted of 480 Spanish students in their 5th and 6th years of primary education and 1st and 2nd years compulsory secondary education (40% females and 60% males). Several tests were administered: a PROESC subtest, EVAPROMES and story writing. In addition, we collected the marks obtained in the previous year for the subject of Spanish language. Results: Spontaneous writing product quality was related to academic achievement in Spanish language, and metacognition, as measured by EVAPROMES, was the underlying variable that explained differences in the difficulty of the quality indicators. Discussion and conclusions: The differences observed in an analysis of indicators by educational level (primary vs compulsory secondary education) appeared to be due to the methodology employed by teachers in class and the standards required. Implementing spontaneous writing tasks that involve the application of metacognitive strategies (meta-writing) would exert a positive impact on students' academic performance.