Children who have symptoms of Attention Deficit/Hyperactivity Disorder (ADHD) often experience disruptive and off-task behavior and lower school performance than would be expected based on their cognitive abilities. These behavior and achievement difficulties are a challenge to teachers, who often provide inclusive education for children with ADHD. This study explored whether a self-monitoring intervention can help children with ADHD-symptoms to reduce off-task behavior as well as improve their cognition. The participating children were seven boys in special needs education with an age between nine and twelve. The students used an interval timer to remind themselves to monitor whether they were still on task. During math classes, observations, teacher ratings and neuropsychological tests were assessed. The results showed that off-task behavior was significantly reduced during the period the interval timer was used compared to baseline (reduction from 46.8 to 27.3%), as measured by observations (effect size: η2p = .83) and this was confirmed by teacher ratings (effect size: η2p = .69). With respect to cognition, children only showed significant improvements in inhibition (effect sizes: Cohen's d = 2.62 and 1.24). The teachers as well as students evaluated the intervention mainly as positive. In line with previous studies, we found that that a self-monitoring intervention can be beneficial for children with ADHD-symptoms. Larger studies including a control group and blind observers are necessary to establish these results and to investigate the underlying mechanisms.