We built Bia to provide affordable school refusal treatment. You can purchase it yourself or you can ask if your school is able to get it for you.
School will often time offer support at different teirs. I'd start by talking to them and depending on the severity of school refusal they can provide resources to read, counselors to speak to, outside therapists to get help from, or even alternative schools or placements. This site also offers some self-guided resources for you and your child.