The most common and most likely answer to this crossword clue is the 12 letter word KINDERGARTEN
I've seen this crossword clue in The Daily Telegraph
K
I
N
D
E
R
G
A
R
T
E
N
More clues leading to the result KINDERGARTEN
- Nursery school, more gentle, cultivated great name
- Darn green kit needing repair for school
- Green art evolved first sort of nursery
- Infant school class painting in splotchy green
- Nicer information about pictures of childcare facility
- Family regard mobile number necessary for preschool
- School rent gear out by type
- Uneasy entering dark preparatory school
- School serves up great dinner to knight
- . More helpful drawing-in information for a children's class
Buddy explains!

The straight part
We know that "School" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case school = kindergarten.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 KINDERGARTEN mean?
KINDERGARTEN noun- An educational institution for young children, usually between ages 4 and 6; nursery school.
- The elementary school grade before first grade.
- The two levels between nursery and prep; the second and third years of preschool.