When you delete a schema and there are instances of classes, the delete can fail with an exception from inverse maintenance. This is sub-optimal given the objective is to discard all the schema defined object instances as well.
Little useful information about the exception is reported at the user interface (message box, jommsg log, examples for 1310 exception are attached). So it can take a considerable amount of time to unravel the relationships and script the necessary deletes (class and property names involved when the exception is thrown would help).
But nothing would help as much as simply deleting the schema and the data.