From BBC http://www.bbc.co.uk/news/education-29342539 :
Low-level, persistent disruptive behaviour in England’s schools is affecting pupils’ learning and damaging their life chances, inspectors warn.
The report says too many school leaders, especially in secondary schools, underestimate the prevalence and negative impact of low-level disruptive behaviour and some fail to identify or tackle it at an early stage.
Source: Poll conducted by YouGov for Ofsted, http://www.ofsted.gov.uk/news/failure-of-leadership-tackling-poor-behaviour-costing-pupils-hour-of-learning-day
This is one of many low-level school issues that affect undergraduate mathematics teaching. In a mathematics lecture, weaker students are more prone to “loosing the thread” than in most other courses. Also, students for whom English is not the first language, in particular, most from overseas are more sensitive to the signal-to-noise ratio than natives, and, at a certain level of background noise, their understanding of the lecture becomes seriously degraded. In my opinion, this is one of many neglected issues of undergraduate mathematics education. I in my lectures always insist on complete silence in the audience (and usually start my first lecture with a brief explanation of the concept of signal-to-noise ratio).