Training Ship Indefatigable was a British training school for boys intending to join the Royal Navy or the Merchant Navy.
Until the middle of the nineteenth century the British Merchant Navy had no recognized training schools for boys entering the service. Education consisted of boys about 15 years old going to sea “to be led, guided, bullied and socialized into the culture of the sea”. There was no distinction between training for AB, and the training of future masters. Through experience it was possible to rise to the position of Master without any formal training. Beginning in the mid- nineteenth century various forms of navigational and seamanship schools were created to remedy the problem.
Referenced from Wikipedia – read more here
Also – see the Old Boys Association.