Child support over 18 if custodial parent voluntarily held child back a school year in order to collect for one more year.

I have a son that i paid child support for 17 years he turns 18 in march of 2010 next year but will not graduate next year but due to his mother held him back repeating 6th grade even do he had passing grades and at that time i object to her holding him back, i did not think of it at that time but know 8 years later i see that see will get and is asking the court to extend the support for one more year. Question do I have any chance not to extend the support past 18 if I could show the court that she voluntarily held him back, and why should I be penalize for one more year if she made decision on her own.