This intervention study examined the writing abilities of 1st (ages 6-7) and 2nd (ages 7-8) grade students in the Basque Autonomous Community, utilising Basque, Spanish, and English. We compared two distinct teaching methodologies: the PYCTO methodology, which employs a multilingual approach for teaching writing across the three languages, and the traditional monolingual approach, focusing on one language only. 873 compositions written in these languages were analysed using quantitative methods. Lexical richness was evaluated using analytic measures, while holistic measures assessed overall writing competence in four dimensions. The results indicated that students taught using the PYCTO methodology significantly outperformed those taught with the traditional monolingual approach in both 1st and 2nd grades. This finding underscores the effectiveness of multilingual teaching methods over the conventional monolingual approach, suggesting that integrating various known languages in teaching offers substantial benefits in language learning.