If there's a slight variation in your name between your government ID and school marksheets, it's best to use the name exactly as it appears on your 10th or 12th marksheet while filling out the CAT form, since academic records are the primary documents used for verification during admission. However, ensure that the variation isn’t too major if it is, you might consider getting an affidavit or correction done in the future. For now, stay consistent with your academic records to avoid any mismatch during document verification after the exam.