Students passed in 2012, this year can appear in CBSE Improvement Exam 2013 only if they are passed in 2012 and only in those subjects which you passes. Forms will be available from August.
There is some confusion whether the new change in rule by our Capable and Educated committee will accept the the cbse improvement exam marks in 2013 combined examination ISEET. They should accept the improvement marks there is no reason to deny it but I have asked them for the complete information and will post update as soon as they give a reply.
For further details about CBSE Improvement Exam read : http://www.improvementexam.com/cbse-improvement-exam and http://www.improvementexam.com/what-is-cbse-improvement-exam