You can always study asa private candidate while service. But then ine even dont need to do this Army has an inbuilt program for soldiers through which they can 'upgrade' their education. In short, you study as you would do as student but of course you have to do your job too. You can then appear in the exam as a regular candidate. The only difference is that all this done through the Army. Though preference or relaxation is offered. Bottom line, you can improve your education while being in the army and then apply for ISSB provided you are still within the age limit - which is more in case you are applying as a soldier.