All the user fields that are valid in an upload file are listed below, except for any custom fields you may have created (for which see below.)
firstname,lastname,username,email,password,auth,idnumber,institution,department,city,country,timezone,lang,mailformat,maildisplay,maildigest,htmleditor,autosubscribe,skype,msn,aim,yahoo,icq,phone1,phone2,address,url,description,descriptionformat,interests,oldusername,deleted,suspended,alternatename,lastnamephonetic,firstnamephonetic,middlename

Format of csv file that site admin can use to impost students and other users:

username firstname lastname email
UN1 FN1 LN1 s1@example.com
UN2 FN2 LN2 s2@example.com
UN3 FN3 LN3 s3@example.com