The author says

The vertical spread between the lines indicates between classroom variation in terms of intercepts and is consistent with a random intercept model

Similarly on 115-116 the author says

The variability in the level of the classroom means differ between girls and boys; that is, the intercept variance may not be constant for gender. Most noticeable is the appearance of possibly two groups of boys—those with high segregation indices and those with lower values

http://courses.education.illinois.edu/EdPsy587/GLM_GLMM_LMM.pdf