The Class improvement scheme or exam rules change from state to state. In CBSE it is very clear, you can take it only after one year.
The Maharastra state board gives students two chances(for instance the Feb-March batch, can apply for October and then once more in Feb-March if they are not happy with the first improvement exam). Such details are not very clear on the GSEB.
Some boards will ask for the candidate to sign a letter that he/she will not enroll for higher studies.
Most people will re-appear as private candidates in a chosen subject. If you want to re-appear for all subjects, then you have to be a regular student, so please discuss with the school administration office/Principal as to whether they will take you.
Some students also change the board in the hope of getting better marks. So, please contact your Principal and also please contact the Gujarat State Education Board (07923248461).
Betterment or Improvement exams means loss of atleast 1 year of precious time, since you have mentioned that you are a current student, please focus on the studies. Marks alone at state board are not very important unless it is for getting a government job after 10th/12th. People will average marks do well in the entrance exams for the various colleges/course and then shine in the graduation/degree education.