It's completely up to you..& you have to identify the reasons why you scored bad otherwise you won't be able to recognize your limitations. Identify your faluts & work smart on those. As I can say mine was around 66% in half yearly of XI & 75% in annual but I wasn't depressed 'cause I knew that I recovered well. And bro life is meaningless without challenges. I'm right now pretty confident about my destination. I should say u need to keep up cause winners never quit,they carry on.

Half Yearly Exam does not determine Your Future , Marks at school level exams are completely different from the perspective of Entrance exams, In Board they not only check Concepts but also Your writing skills, How You can present the answer in best way Possible but for Entrance exams You need strong Problem-solving skills and Very good Conceptual Knowlege Now You have more than Two years You can Improve them and still dream for IIT.

The approach for academic exams and for iits and nits is bit different. There are people who get very less grades in academic yet top their jee mains and advanced exams. If you want to get admission in better colleges after your 12th better prepare for jee mains and advanced. The ncert textbooks for your academic purposes are also useful to prepare for jee mains. So you can do well in both the sections.