Air India luggage rules

So I’ve booked a flight but it wasn’t available for student ID so I booked it normally through Air India Though the limit is 15 kgs I believe my luggage may exceed that limit as it can be 18-20 kgs Indigo always provided me with student discounts for upto 25 kgs So will there be any issue at the baggage counter, like extra amount or they can let it go because many a times Indigo staff is chill regarding luggage limit.. no idea about Air India