The most common and most likely answer to this crossword clue is the 10 letter word SCHOOLDAYS
I've seen this crossword clue in The Guardian
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More clues leading to the result SCHOOLDAYS
Buddy explains!
The straight part
We know that "They should be happiest!" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case they should be happiest! = schooldays.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 SCHOOLDAYS mean?
SCHOOLDAYS noun- A day during which the public schools are open, usually synonymous with weekday during the school year.
- The part of a day when students attend school.
- The period of one's life when one attends school, particularly, primary school (as opposed to college days); one's youth.