Populate a select box with data stored in the database?

You should import the schools as a table in the database, separate from the other WordPress tables.
You can than write a function that selects and returns schools and eventually link it to the field via ajax using something like select2 or jquery autocomplete.

Hata!: SQLSTATE[HY000] [1045] Access denied for user 'divattrend_liink'@'localhost' (using password: YES)