Legal Question in Family Law in California

It would be easy to say no, but the better form would be to make that final payment and then seek a modification of the support order based on the changed circumstance. As the graduation date is most likely in May or June, you have time to file for this modification so prior to the actual graduation event.

I am 99.9% sure you are mistaken about your support order. The statute provides that support is payable until BOTH turning age 18 AND either completion or dropping out of high school, OR turning 19. It requires a judicial determination and findings of fact that compel deviation from the statute for the court's support orders to deviate from that rule. I have NEVER seen it done. So if your son is not yet 18, you will have to pay support until he turns 18, regardless of his graduating. As for the core of your question - which really is whether you pro rate the final month's payment, the answer is "no." The fact that you pay in installments does not mean you don't pay the full month for the final month support is due. It's not like renting an apartment. Although I agree it should be, so much of child support is now collected and distributed by the Dept. of Child Support Services and the bureaucrats don't want to have to be dealing with partial payments all month.