The username field in SAKAI_POSTEM_STUDENT is varchar(36).
If you have a Sakai user with an eid longer than 36 characters (e.g. a guest user with a very long email address), then uploading a spreadsheet containing that username causes a mysql write failure:
caused by: com.mysql.jdbc.MysqlDataTruncation: Data truncation: Data too long for column 'username' at row 1
The field should be redefined to be varchar(255) in StudentGradesImpl.hbm.xml and a conversion script included.