When adding Inter-District data for past years, the following error may be encountered: 

When attempting to add a new Inter-district enrollment record, the date is validated against the currently logged in school year. That year is loaded into memory when the Students form is opened. If the logged in school year is changed and the Students form is not closed and reopened, the original year will still be used for date validation.

The Students form needs to be closed and reopened to re-set the year value used for validation.