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Save the file and go into quick import and use the "suggest field map" and it should pick the fields for you automatically. Hit the "Import" button.
If you are saving your file as a .csv (comma delimited text file), you may need to change the delimiter field on the quick import screen to 'Other' and a ',' in order to prevent errors.
You can check the box to exclude the 'first row' so your headers do not error out.
As long as all your data and student numbers are correct, you can select 'Update the student's record with the information from the file being imported' under the advanced import options.