To do the Data import navigate to:
  1. Core
  2. Settings
  3. Data
  4. Data Import
  5. Transfer School Import

What is the difference is between the transfer_school_type_desc and the transfer_school_type_id?

In the File Layout Help click Sample File, this shows the 2 items you will want in the file (only the School Name is Required)  the transfer_school_type_desc shows what options are available (Private, International, Home, Public, Charter, Parochial) they type needs to be written out this way exactly in the CSV file, and transfer_school_name is the School Name as you want it to display (again this is a required field)

You can run a test file with one school name and School Type to see what Data this imports (you can always remove this information) 

Once imported, the checkboxes on each School name imported for both of these items below will be selected:
"Include this school on inquiry forms to specify as a current school"
"Candidates can choose to attend this school when declining an acceptance"