Yes, you can give the 12th science gseb exam as an ex student prallely doing diploma course as it depends in the school that they are allowing to fill the 12th science board exam form . If the school is alowing to give the 12 th science exam then it possible even some of my friend had given 12 th science exam as an ex student by doing diploma also. So, dont worry it possible but choose the school as they are allowing to fill the gseb board exam form or not.

So , please if you still have any query let me know in comments