It is generally acknowledged that students with special hearing and communication needs should be taught in classrooms with relatively short reverberation times. There has been a great deal of research into the effect of room acoustics on the signal-to-noise ratio experienced by pupils. There is, however, much less research into the effect of class size on ambient noise levels during lessons, and hence on the signal-to-noise ratio and on absolute sound levels in classrooms. These absolute levels are important because many students with special needs are sensitive to high noise levels, either for psycho-acoustic reasons or because of the limited dynamic range of cochlear implants and other assistive devices. In his work as an expert witness on acoustics for Special Education Needs and Disabilities Tribunals, the author has measured in-lesson sound levels for different types of teaching and learning activity in many schools throughout England. He has analysed these to investigate the correlation, when corrected for acoustic factors, between class size and sound levels generated by teachers and pupils, and to consider whether there is a typical class size at which an effect analogous to the Lombard Effect occurs. From this the author draws conclusions as to the acoustic justification for teaching pupils with special hearing and communication needs in smaller classes.