You must sign in to your 18+ web account on the Transport for London website and confirm that you are still eligible. Your confirmation will then be sent to the University of Westminster for approval. You must do this by 1 November at the start of each academic year.
If you fail to re-confirm by this date your student-rate travel concession will be stopped, which means you will have to pay adult rate. TfL is not liable for any expenses, including travel costs, incurred as a result.