The increasing presence of technologies at school has triggered a vivid debate on the way ICT influences students' learning process. Using PISA 2018 data for 15-year-old students and hierarchical linear models, we find an inverted U-shaped relationship between ICT use at school and students' performance in mathematics in 22 OECD countries. In all cases, the excessive use of technology is associated with a lower academic performance, although this penalty differs across countries, which points to the importance of addressing country-specific analyses. The differentiated profile of those very intensive users, who suffer from above-average bullying exposure, draws into question whether the effect can be deemed as causal. Based on Inverse Probability Weighting techniques, the findings indicate that the very intensive use of ICT at school causes an underperformance of students equivalent to around half an academic course in Estonia, Finland and Spain. The results highlight the need for policy makers and instructors to ensure that the frequent use of ICT at school does not interfere with students' learning process.