This study finds out that the phenomena of extremely large classes are still quite common in the hinterland of China，with class sizes constantly breaking official limits. For instance，in Zhumadian city and Nanyang city located in Henan province，the largest classes in lower secondary schools exceed 150；in Ankang city of Shaanxi province the largest classes in primary schools reach 90. The class size prevailing in schools located in county seats is usually larger than those prevailing in rural and urban areas by a large margin. The underlying causes of this phenomenon lie in：a large number of pupils from rural area enter schools in the county seats，and the local governments suffer from inadequate financial resources and connive at the behavior of schools admitting more pupils so as to reduce the cost of education. The existence of super-large classes is not only detrimental to the quality of education，but also harmful to the physical and mental health of both teachers and pupils，creating hidden dangers in school yard safety.