Just food for thought. But, in Missouri, having an appointment, is truly not "in progress". It is perceived as being ok, but it is not. When I first cam to this job, I battled this. On more than one occasion, the child missed the appointment. By then, the child is in school, and it is pretty difficult to back track and kick them out then. I have made great gains at my schools. Not perfect yet, but 3 of my 4 schools, the students do not enroll until I say their immunizations are up to date.
One thing that has really helped me, is that Missouri has a database called, "Show-Me-Vax". It has saved me and parents countless hours of tracking down lost immunizations (and has gotten students in school very quickly). I encourage you to check and see if your state has one. When I first started, my local Health Department assisted me with the right connections to get a pass word.
To encourage you to know how to deal with difficult Principals. Hang in there. Be consistent, honest, and keep on keeping on. It sounds like you are making big differences in the lives of your students and they are lucky to have you. :)