The most common and most likely answer to this crossword clue is the 14 plus 11 letter word PARENT-TEACHER ASSOCIATION
I've seen this crossword clue in The Guardian
P
A
R
E
N
T
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T
E
A
C
H
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R
A
S
S
O
C
I
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O
N
Buddy explains!
The straight part
We know that "School friends" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case school friends = parent-teacher association.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.