It is widely accepted that the indoor environmental quality may strongly affect human’s health, comfort, and productivity. This is especially true in case of school environments, as children are physically still developing and, in comparison to healthy adults, will suffer the consequences of a poor indoor environment earlier. Consequently, IEQ in schools is a very investigated topic also due to the close relationship with building energy performances. From the thermal comfort perspective, the assessment of comfort conditions is usually carried out by means of the PMV and the PPD indices. Unfortunately, the peculiarities of school environments (e.g wide classrooms, the absence of HVAC systems, the kind of activity and clothing) require high care to obtain a reliable analysis. Based on a large experience since almost 20 years by the Italian research team InEQualitES (Indoor Environmental Quality and Energy Saving), this paper is addressed to the main criticalities related to the thermal comfort assessment in schools. Particularly will be discussed the calculation of a sole PMV/PPD value to be attributed to the same classroom in the presence of more measurement positions, the effect of insulation value and, finally, how PMV index has to be used in non-air conditioned environments.