The current study examined the effect of an online relational skills training program, strengthening mental abilities with relational training (SMART), on scores on standardized classroom diagnostic assessments. Several previous studies suggested that this novel intervention has a reliable impact on intelligence scores and some domains of cognitive ability relevant in the educational setting. The current study employed a single-blind design and a matched control group in the delivery of the SMART intervention in 45-min sessions 1–3 times per week over approximately 10 weeks, with students completing the intervention within two weeks of each other. Standardized classroom diagnostic assessments were delivered pre- and post-intervention. The results showed that, controlling for baseline scores, the intervention group performed significantly better than the controls in the mathematics and reading domains post-intervention. Gains in both reading and mathematics were large and significant from pre- to post-intervention for the treatment group, but modest significant gains were also found for the control group.