Son (14 years old) still had whooping cough in April. Cough remains. T…

Son (14 years old) still had whooping cough in April. Cough remains. Today he coughed up until vomiting during class, as a result, the class teacher called and said to come and pick him up. She also wished for his recovery and said that a medical certificate would be needed. Nothing hurts much, but now I can't take my child to school for this reason? Also, how to ease the cough specifically at school? There is no medical personnel at school.