That happens when the source of the attribute is misconfigured, or the entity_type of that attribute is not set.
Search in your database, in the table eav_attribute your attribute by attribute_code and take a look if the entity_type is correct (for example, catalog_product, in case of product attributes).
SELECT entity_type_id, backend_model, source_model FROM eav_attribute where attribute_code = 'school';
If that is correct, it might me a misconfiguration of the Source, check the source_model is correct.
