Eton College:

A boy who is late for any division or other appointment may be required to sign "Tardy Book", a register kept in the School Office, between 7.35am and 7.45am, every morning for the duration of his sentence (typically three days). Tardy Book may also be issued for late work. For more serious misdeeds, a boy is summoned from his lessons to the Head Master, or Lower Master if the boy is in the lower two years, to talk personally about his misdeeds. This is known as the "Bill". The most serious misdeeds may result in expulsion, or rustication (suspension). Conversely, should a master be more than 15 minutes late for a class, traditionally the pupils might claim it as a "run" and absent themselves for the rest of its duration.

Please answer a question about this article. If the question is unanswerable, say "unanswerable". What term is used if a master is more than 15 minutes late to a class?
a "run"