One of the biggest challenges that teachers face is the mixed-level classes, where students have different abilities, backgrounds and interests. All classes can be defined as 'mixed' since no student is the same as the other and this makes it difficult for teachers to meet each student's needs. The purpose of this study is to identify this problem by using Lev Vygotsky's 'Zone of Proximal Development' (ZPD) theory as a background and suggest solutions for teachers by making use of Vygotsky's ideas such as peer tutoring and scaffolding. While the starting point of the problem is English language classes at universities in Turkey, suggestions offered for effective teaching are in a more global context. (C) 2015 The Authors. Published by Elsevier Ltd.