Hello

Admission in Thapar is not based only on 12th board marks , they need  your JEE Mains score as well. 12th board score is just taken for reference if you have low jee Mains score , that is your PCM score should be more than 80% and then you have the opportunity to get a seat in Thapar but that also from the remaining seats not your desired seat. Also you can upgrade to any branch later in the starting of second year through your CGPA.

HOPE THIS SOLVES YOUR QUERY.
GOOD LUCK!!