Assistants serve an essential role in special education to support children with disabilities, but they should be properly trained and supervised. The coaching approach represents one trend that has been gradually implemented in occupational therapy (OT) and rehabilitation services. Still, few studies clearly define the coaching intervention, measure the fidelity of coaching practices, or evaluate capacity building of the caregivers in the long term. This quasi-experimental study compared one-on-one coaching in natural environments following a workshop with a training workshop. Both public schools do not have regular OT services. The primary outcome was the assistant’s performance, measured with the Goal Attainment Scaling (GAS). The secondary outcome was the fidelity of coaching implementation, measured with the Coaching Practices Rating Scale (CPRS). The GAS showed an increased performance of the assistants after the intervention, with significant differences between groups post-intervention (p = 0.015) and large effect size (r = 0.55), but no long-term significant improvements were found at the follow-up (p = 0.072). The CPRS showed an adequate implementation of the five coaching components (joint planning, observation, action, reflection, and feedback), with a total score of 3.5 ± 0.72 (mean ± SD). The results suggest that coaching sessions provided by OTs in schools may improve assistants’ skills to facilitate the student’s participation.