School demanding sign of mother but I we have divorced, what to do

Dear sir/ madam I m a single father of my son since his birth as my wife had taken divorce under section 13 b after birth of child.she had written permanent custody in my favour. I m loozing after my child since his birth. Now he is going to appear in 10 cbse exams.school authorities called me to sign on his exam performa. But there is option for signature of both parents. I told them to leave box of mother signature blank.because she is not in our contact. School authorities are saying that it may cause some objection from board side. What should I do? I hv no objection on mentioning his mother name but her signatures not in my hand Pls reply on priority. Thanks