The most common and most likely answer to this crossword clue is the 14 letter word ANTILOGARITHMS
I've seen this crossword clue in The Guardian
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Buddy explains!
The straight part
We know that "numbers in old-fashioned school lesson" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case numbers in old-fashioned school lesson = antilogarithms.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does ANTILOGARITHMS mean?
ANTILOGARITHMS noun- The number of which a given number is the logarithm (to a given base).