Framework: Internet addiction and simultaneous preference for online social interaction is associated with high levels of depression symptoms and anxiety, and its excessive use is harmful with regard to academic performance. The study aimed to identify the variables that influence the mental health of students in primary and secondary education; and to analyse the influence of the variable "addiction to the internet" on their mental health. Methodology: Descriptive, correlational and analytical study, with a sample of 673 students attending primary and secondary education in a city in central Portugal. They are mostly girls (52.0%), with a mean age of 15.05 years (+/- 1.906 years). For data collection the sociodemographic and academic characterization questionnaire, the Mental Health Inventory [1] and the Internet Addiction Test of Young [2] were used. Results: The majority of the students live in urban areas (64.9%), are attending basic education (59.7%), without repeating previous years (72.2%), mostly have a father with basic education level (45.6%) and a mother with secondary education level (41.5%). Boys prevail with a higher impact on addiction to the internet (59.0%), students aged from 16 to 17 years old (41.6%), living in urban areas (66.7%) and attending secondary education (63.5%). The primary school girls residing in rural areas who have never had to repeat previous years are the ones who display more psychopathological symptomatology. The lower is the loss of control the lower the negative impact of the Internet addiction in the daily life. Students with low academic performance have a higher level of internet addiction (46.8%) and more psychopathological symptomatology with significant differences on the overall school performance (p = 0.001). Conclusions: The extension of mental health problems related to the excessive and problematic Internet use ensure the need for more intervention, both at school and within the family, in order to increase self-control and minimize the negative consequences.