If you Master of Death badge is missing, then you have not learned all of your school's spells. There are 3 people who have Death spells to learn from other than your teacher. The secret trainer in Collusus Blvd, the Death Tree in Nightside, and Craaky(?) in Maryleybone.
I had the same problem. The last spell I needed was from the girl in the fair grounds in WC. It's only the -80% death shield but it counts as a death spell. Once I got it I got my badge.
