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.
This post may help you to understand and fix your attribute (re)source model.