ISRO cannot take direct 12th students. You must become a graduate. You can apply for IIST Thiruvananthapuram. Here UG students are directly recruited for ISRO till now. But unfortunately there is a case in progress where public sectors should not recruit from colleges and they should recruit only by GATE Score. The rule may be finalized within the four years of Engineering. But still IIST gives very good exposure to ISRO. IIST has very limited seat for UG. Even if you don't get, it doesn't conclude you cannot get ISRO. All you need to do is work hard during your graduation. Have good projects and intern.

You to join IIST to get absorbed into ISRO