Do you have to tell a law school of previous attended?

Yes, you generally have to tell a law school about any previous law school attendance. Law schools require complete and accurate information on applications. Failing to disclose prior attendance is a serious breach of ethics and could result in serious consequences, including revocation of admission if discovered later. This is true even if you didn't graduate or complete a significant portion of the program. You need to report all attempts at legal education, regardless of outcome.