The most common and most likely answer to this crossword clue is the 5 letter word SHOAL
I've seen this crossword clue in The Times
S
H
O
A
L
More clues leading to the result SHOAL
Buddy explains!

The straight part
We know that "School ably showing its alternate form" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case school ably showing its alternate form = shoal.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 SHOAL mean?
SHOAL noun- A sandbank or sandbar creating a shallow.
- A shallow in a body of water.
- To arrive at a shallow (or less deep) area.
- To cause a shallowing; to come to a more shallow part of.
- To become shallow.
- Shallow.
- Any large number of persons or things.
- (collective) A large number of fish (or other sea creatures) of the same species swimming together.
- To collect in a shoal; to throng.