You can check whether you need an ATAS certificate at the government's ATAS page.
You are issued an ATAS Clearance Certificate for a specific course with a named provider which covers the length of that course from the course start date to the course end date given on the CAS, plus up to an additional 3 calendar months of study. The certificate remains valid as long as your provider and / or course details do not change. If your provider and / or course details do change, or your course end date is postponed for a period of more than 3 calendar months, you must apply for a new ATAS Clearance Certificate within 28 calendar days of being notified of the change in course, regardless of when your leave expires.
You will also need to apply for a new ATAS certificate if:
Where there is a change to the length of your course because you are deferring your studies, and sponsorship is withdrawn, the application for a new ATAS clearance certificate should be made at the time you are applying for entry clearance or leave to remain in order to start the course again.