There's no publicly available record consistently tracking which school "tops" the ICSE or ISC board exams in Kolkata, or indeed anywhere in India. The boards release aggregate results, often showing the top percentage of students achieving distinctions, but they don't declare a single "top" school. Rankings vary depending on the criteria used (e.g., percentage of students scoring above a certain mark, number of students scoring top marks, etc.) and are often generated by third-party organizations, not the boards themselves.
Therefore, while Ramakrishna Mission High School, Kolkata, is a highly reputed school, there's no definitive answer to whether it has ever officially been declared the "top" ICSE or ISC school. It consistently performs well, but "top" is a subjective and fluid title.