When Bruce returned to Wayne Manor with the three Robins, Thomas was arguing with Nightwing.
The situation was roughly this: the time Thomas spent talking with Nightwing had clearly not been enough for Dick to finish his French homework, so when Thomas went upstairs to check it, Dick was doomed.
Dick had not completely neglected it, really. It was just that what he had written had been scribbled down while he was so sleepy he could barely keep his eyes open. That was even more fatal than not writing anything at all, because it masterfully displayed his French proficiency—Dick had successfully brought France and the British Empire back into shared prosperity at the end of the twentieth century.
Thomas was furious after reading it. He felt that he had tried very hard to suppress his anger and speak properly to Dick, but Nightwing thought his demands were too high. Dick was so sleepy he could barely open his eyes; the most urgent thing was to let him sleep, or it would practically be abuse. Thus, the two began arguing.
Going off-topic was an indispensable part of arguing. As they argued, the subject somehow shifted to the relationship between ballet and French.
Nightwing felt there was no necessary connection between learning ballet and learning French, so there was no need to study them together. Even if he wanted to learn French, he did not necessarily have to learn ballet. Thomas's line of reasoning, meanwhile, was, "You don't know ballet either? Then learn that too."
In fact, there was no reasoning with a patriarch like Thomas. In the end, every line of logic returned to the eternal argument of "Are you the father, or am I the father?" Nightwing naturally could not possibly think he could be Batman's father's father, so he