© 2020 by the authors. Licensee MDPI, Basel, Switzerland. (1) Background: Test anxiety has been found to negatively affect students’ mental health and academic performance. A primary explanation for this is that anxiety‐related thoughts occupy working memory resources during testing that cannot be used for test‐related processes (such as information retrieval and problem‐solving). The present intervention study investigated whether physical activity could decrease anxiety levels and improve maths test performance in sixth‐grade children. (2) Methods: Sixty‐eight children of 11–12 years from two primary schools in New South Wales, Australia were categorised as low or high anxious from their scores on a trait‐anxiety questionnaire. After this assessment, they were randomly assigned to the activity break condition, in which they had to do several physical activities of moderate intensity (i.e., star jumps) for 10 min, or the control condition, in which they played a vocabulary game for 10 min. The outcome measures were children’s anxiety levels at the beginning, during, and at the end of the test, invested mental effort, perceived task difficulty and maths test performance. (3) Results: Results showed that regardless of the condition, low anxious students performed better on the maths test than high anxious children. No differences were found for any of the variables between the activity break condition and the control condition. (4) Conclusions: Although test anxiety was not reduced as expected, this study showed that short physical activity breaks can be used before examinations without impeding academic performance.