I have no knowledge of the ZOE app so cannot offer an informed view on it. However based on your description of its functionality, it would appear that there is no sharing of personal data between school and ZOE. (Truly) anonymised data is not defined as personal data and therefore processing such data has no compliance implications from a data protection perspective. Based on your outline, the school will not be acting as a data controller in this situation as only ZOE will be processing personal data. The only advice I would give is that if a school is recommending an APP for parents to use (particularly which this may lead to the parents sharing personal data with the APP) then the school would need to have a high degree of confidence that the APP is acting in a manner that is ethical and safe.
