Which AR contains the Civilian Education System policy?

There isn't a single AR (Army Regulation) that solely contains the entire Civilian Education System (CES) policy. CES policy is spread across multiple ARs and supporting documents. The specific regulations will depend on the aspect of CES you're interested in (e.g., tuition assistance, education services, etc.). You'll need to consult the Army Publishing Directorate website or your installation's education center for the most current and relevant regulations.