To add a destination school please go to Admin > Config > Destination School Setup.
Once in this area double check that the school you require is not listed, if not then click onto add new.
To add a new school here you will require the School name, DfE number (split up into LEA and ESTAB number) and URN number - if you are unsure of any of these please click here to search the "Get Information About Schools" site for the information.
Once added this will show in the dropdown on the extended tab of all students under Destination school.