This chapter presents the first of three interrelated strategies aimed at realising the plurilingual vision of the Barcelona Summit Agreement (mother tongue plus two additional languages). Under this proposal, modern foreign language (MFL) classes would be harmonised in line with the Common European Framework of Reference for Languages (CEFR) through cross-age teaching. In essence, students would no longer attend MFL classes based on traditional grouping arrangements with other students of their same school year/grade. Instead, they would be grouped with students of different year groups who have the same CEFR level. This chapter discusses various aspects of cross-age teaching in line with the CEFR: the rationale behind the strategy, the expected CEFR standard students should achieve by the end of their second-level studies, how the proposal would work in practice, the benefits of the strategy, how it has the potential to motivate students more, previous research on similar grouping arrangements, the implications of the strategy for students and schools, the importance of school ethos and culture in terms of implementing such a grouping initiative, and finally, the findings from the empirical research conducted with MFL students, teachers, school leaders, parents, and other stakeholders on various aspects of this novel strategy.