Concurrent with mandating completion of Grade 12, the whole atmosphere, teaching methodology, and teacher training, qualifications and evaluation must be improved.
Another issue is, what if students a) are unable to complete grade 12 until they are 28 years old? b) mentally unable to handle grade 12? c) completely and utterly bored to the point of "rebellion"?
The old, "you can bring a "camel" to water, but you can't make it drink."
American School Curriculam in general is one of the worst school curriculam in existance with the exception of the academic institute (prep school, grammer) catering for the upper ushalone of the american society. Indian syllabus fares well in academia but the style of teaching and the skill acquired harms the future generation and make them weak. The key lies adopting the best practicies and best aspects of syllabus from UK or Germany within the cultural contex.