CSky, I haven't worked with &build_select_field_from_db however, using the info you've provided, it appears you are pulling up the School Name by using the $rec{'School'} fieldname for displaying. Have you tried something like this?
&build_select_field_from_db("School", "$rec{'School'} $rec{'City'} $rec{'Country'}")
&build_select_field_from_db("School", "$rec{'School'} $rec{'City'} $rec{'Country'}")