We should probably first deal with the underlying issue that you cannot force your ex-husband to exercise his custodial rights under the possession schedule of your divorce decree. Custodial rights are rights, not obligations. However, his duty to financially support his child is an obligation and should he decide to stick you with more child care requirements then it may be time to look at contacting the OAG or a private attorney to pursue an increase in child support. (The Office of the Attorney General does that work for free so that may be a better option for you.) Perhaps there is even an argument to go above the support guidelines as he is not exercising his visitation periods and not sharing in the cost of child care during those periods.

As far as the "school age" language in the decree goes, let's call that what it is: some terrible drafting. The decree should have been drafted to specify enrollment in the standard public education program or an equivalent private school program. But his attorney didn't do that so you're stuck sorting out the mess. I agree with the response above that courts are most likely to interpret "school age" as consistent with the age upon which a child would be enrolled in kindergarten or a comparable program.