Abstrak
This study aimed to determine whether there was a significant difference in the writing skills of eleventh-grade students at SMA Negeri 1 Nunukan, which used the Merdeka Curriculum, compared to those at SMA Negeri 2 Nunukan, which adopted the 2013 Curriculum. A causal-comparative research design was employed, with a sample size of 114 students selected using a purposive sampling technique, this technique was chosen to ensure that the respondents have characteristics relevant to the research objectives. The research instruments included writing tests and interviews, in which the writing test questions were adapted from Collins' TOEIC Writing Skills. Due to the non-normal distribution of the data, the Mann-Whitney test was utilized to determine the differences between the groups. The results revealed a significant difference in the writing skills between the two groups. The mean score for students at SMA Negeri 1 Nunukan, which implemented the Merdeka Curriculum, was 144.04, placing them at a TOEIC proficiency level of 7 and at the B1-Intermediate level according to the CEFR levels. In contrast, students at SMA Negeri 2 Nunukan, who learned English under the 2013 Curriculum, had a mean score of 122.28, placing them at a TOEIC proficiency level of 6 and also within the B1-Intermediate level but noticeably lower. The Mann-Whitney test showed a p-value of 0.001, which was less than the significance level of 0.05, indicating a statistically significant difference. Furthermore, the effect size was calculated at 0.4312, indicating a medium effect. This disparity in writing skills is attributed to the differences in learning systems, with the Merdeka Curriculum proving more in enhancing students writing skills compared to the 2013 Curriculum.