If an 18 year old drops out of school, can I stop child support?

I pay child support for my son who turned last September 2014. He lives with his greedy controlling (narcissistic personality disorder) mother who does not force him to go to school and also no punishment for the skipping school. He was basically uninvited from his regular high school as a junior in May of 2013, only 3 weeks before the end of the school year, for chronic nonattendance. He then went to another school that teaches smaller classes without the fluff in order to graduate. As of last fall, he was so far behind, his teachers suggested he just go for a GED instead since the credits he would have to make up to graduate normally would take him over 2 years, he would be almost 21 by that time. Again as of today at a parent teacher meeting he has extreme chronic nonattendance, 63% attendance the total school year, only 30% since the beginning of the calendar year . The school, as of tomorrow will be deeming him "officially truant" in a letter to the state DOE. I have asked the school for a dated hard copy of the letter for proof. My divorce agreement reads I am to pay child support to age 18 or expected date of graduation from high school, June 1, 2015, whichever is longest but not past the age of 19. What do I do now? Can I stop child support payments or do I still have to wait until June 1, 2015?