Many if not most child support orders will require child support to continue until the child turns 18 or when the child is finished with high school, whichever is later. However, as you are finding out, the state needs to confirm the child has not graduated early, dropped out, or otherwise. So, the certification of attendance is required. I suppose if this form is not filled out in a timely manner, child support will cease.
This is just my take, but if support does cease and then court action ensue due to the failure of the required party to submit the form, I would definitely request all legal fees be paid by said party. To avoid any such hassles, you may want to make some phone calls and make sure support stays active.