The most common and most likely answer to this crossword clue is the 7 letter word GRAMMAR
I've seen this crossword clue in Independent
G
R
A
M
M
A
R
Buddy explains!
The straight part
We know that "School/rules" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case school/rules = grammar .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 GRAMMAR mean?
GRAMMAR noun- A system of rules and principles for speaking and writing a language.
- The study of the internal structure of words (morphology) and the use of words in the construction of phrases and sentences (syntax).
- A book describing the rules of grammar of a language.
- A formal system specifying the syntax of a language.
- Actual or presumed prescriptive notions about the correct use of a language.
- A formal system defining a formal language
- The basic rules or principles of a field of knowledge or a particular skill.
- A textbook.
- A grammar school.
- To discourse according to the rules of grammar; to use grammar.