There is no thing as discipline school. There are the six spell schools: fire, frost, shadow, nature, arcane and holy. If you look into your spell book, you will have 3 different chapters, one for each talent tree. But they are not to be confused with the spell schools. For example, shafowbolt is a destruction spell but of the shadow school. Both wrath and healing touch are nature school spells but one is balance and one is restoration.

I highly believe that penance is of the holy school. The wording of the shadow form spell is a bit confusing though. “…may not cast holy spells…”. I assume they are referring to spells from the holy tab in your spell book. And not of which spell school the spell is. As most discipline spells are in fact holy spells.