What is the ruling on working in a private school with an internal contract without declaring the work in the residency permit?

If a resident is stipulated to be prohibited from working and accepts this condition, then he must fulfill the condition, and it is not permissible for him to resort to trickery. However, if the condition does not cover all types of work or the work that is permissible for you, then there is no harm in it. You can consult the responsible authorities for clarification.