Assessment is a key aspect of teaching. Assessment is particularly sensitive and demanding in the context of Second or Foreign Language (L2) teaching, involving multilingual students. Under this light, student-centered approaches to assessment require the implementation of alternative assessment techniques, including self-assessment. This research aims to highlight teachers’ perceptions and possible reservations regarding the aspects of the validity and reliability of self-assessment, in the context of teaching Greek as L2 in multicultural public schools. In addition, it investigates teachers’ attitudes about the possible benefits of self-assessment implementation. Research results agree with previous research arguing that self-assessment reinforces students’ self-awareness and self-management. They also highlight individual latent aspects of student performance. At the same time, concerns persist regarding the reliability and validity of the method in a multilingual context. Despite these challenges, the findings confirm that appropriate training for both teachers and students can mitigate these issues and ensure the effectiveness of self-assessment. Specifically, 75% of teachers consider self-assessment a reliable and valid form of evaluation, provided that appropriate support mechanisms are in place. Their insights reinforce the potential of self-assessment to improve student performance while underlining the importance of systematic implementation and targeted training.