The assessment of inattention and hyperactivity/impulsivity behaviors' in schools is made worldwide using observational scales and it is important to recognize their distributions and the best criterion for making an hypothesis of ADHD. The present paper analyses the ratings given by teachers to a sample of 1,239 students (from 6 to 12 years old), using the SDAI scale (Marzocchi, Re e Cornoldi, 2010) for making an hypothesis of ADHD. Three different criteria are compared: the severity of the overall scores (below the fifth percentile), the cut-off sometimes used of a mean/item score of 1.5 and number of symptoms (minimum of six) as suggested by DSM-5. The comparison shows that different diagnostic criteria lead to partly different identifications. The combined use of the fifth percentile and the number of symptoms criteria appears to provide a reasonable estimate (with an identification of 4.6% of children) of the presence of severe symptoms of ADHD in school.