Private School I Graduated From Is Not Giving Me My Certificate That They Promised

So I was forced to go to this private school for autistic people for my 12th grade year of school. At first when I went there I was worried I would not get my diploma because the school wasn’t giving me the credits I needed but they told my mother I would get a homeschool certificate that works the same as a diploma so I wasn’t worried. But now that I have finished my school year there they have yet to give me my certificate it’s been almost eight months since I’ve been there and every time we say something to them they either ignore us or just say that they will send it to us but never actually do. They are lying to me and my mother and I have heard other students that also went to this school say that similar things has happened to them too. I’m not sure what to do about this can I have some advice.