J. B. Kriplani, whose full name is Jivatram Bhagwandas Kriplani, was indeed a teacher by profession before he turned into a full-time political activist. He taught at several prestigious educational institutions, including Benaras Hindu University. This sets him apart from the other options in the question.

Option 1, A. N. Sinha, and option 2, Braj Kishore Prasad were notable figures in the freedom movement and were close associates of Mahatma Gandhi, but they were not teachers by profession. A. N. Sinha was a lawyer and politician while Braj Kishore Prasad was a lawyer involved in the Indian independence movement.

Option 4, Rajendra Prasad, went on to become the first President of India. He, too, was a lawyer and a key leader in the Indian National Congress, but he also was not a teacher by profession. Thus, option 3, J. B. Kriplani, is the only correct answer to the question asked.