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.