There are many schools with special classes. Most of them are for children with Autism. These classes come with a large amount of administrative work and usually a principal will delegate this to the Deputy Principal, who, more often than not, will also have full-time teaching duties.
My suggestion is that any school with any type of “unit” (I don’t like this term but it means in this case, 2 classes for any additional need) will have an Administrative Principal – that’s 2, 3 and 4 teacher schools to be added to the 5+ teacher schools where this already is the case.
However, for schools where there are 12+ teachers, the deputy principal should also be administrative, if there are special classes in the school.