The letter is best if you keep it short. I suggest hand-delivering 3 copies of the letter, one for the classroom teacher, one for the principal and one for someone higher up in charge of special ed. This is what the letter needs to say:
Dear Ms. School Principal: I am writing to request a multi-disciplinary team evaluation for my daughter, Name, a student at School Name. This letter is my written consent for evaluation. As you know from our meeting on Date, Teacher Name reports that my daughter is distracted in class, will not speak, bites herself, touches herself and is not meeting cognitive or academic goals in spite of repeating first grade. Therefore my daughter needs a full developmental evaluation by the school psychologist, social worker, speech therapist and occupational therapist (maybe physical therapist, too, depending on other reported symptoms). Thank you for your time and consideration. Signed, Parent Name
Is it ok to hand deliver? I thought they needed to be sent by registered or certified (not sure which) mail with signature requested. The reason I thought that was because then the school can not deny receiving it. I might be off, though, so I am just asking.