As per my information through CBSE office forms for "IMPROVEMENT" exam will be available in month of October and exams will be held next year in the month of march for your selected subjects and results will be declared with normal 12th aspirants. Remember you should be a droper i.e you should not doing your higher studies to be eligible for improvement exams or you will be declared ineligible for the same.

Dear,

The date has been declare and are as follows:

• CBSE Improvement Exam Form 2018 Dates for Class 10 & 12 will be on 1st week of July.
• CBSE Improvement Form 2018 Last Date for Class 10 & 12 will be on 2nd week of July.
• CBSE Class 10 Improvement Exam 2018 Date will be on 3rd week of July.
• CBSE 12 Improvement Exam 2018 Date same on 3rd week of July.

And the result will be on August of 2018. The date has not been declare.

Hope this will help you.

ALL THE BEST.