Children with mental disorders like Attention Deficit Hyperactivity Disorder (ADHD) often experience challenges in school as they struggle to maintain their attention. Based on empirical studies conducted in school contexts and together with teachers and ADHD domain professionals, we identified design criteria in relation to three core components (sensing, recognizing, and assisting) for designing real time assistive technologies for children with ADHD. Based on these design criteria, we designed the Child Activity Sensing and Training Tool (CASTT), a real time assistive prototype that captures activities and assists the child in maintaining attention. From a preliminary evaluation of CASTT with 20 children in several schools, we and found that: 1) it is possible to create a wearable sensor system for children with ADHD that monitors physical and physiological activities in real time; and that 2) real time assistive technologies have potential to assist children with ADHD in regaining attention in critical school situations.