Few odd classroom blunders indicating something bad could happen at any moment (5,2,8)
I believe the answer is:
sword of damocles
'something bad could happen at any moment' is the definition.
I can't judge whether this defines the answer.
'few odd classroom blunders' is the wordplay.
'blunders' indicates anagramming the letters.
'few'+'odd'+'classroom'='fewoddclassroom'
'fewoddclassroom' with letters rearranged gives 'SWORD OF DAMOCLES'.
'indicating' is the link.
Can you help me to learn more?
(Other definitions for sword of damocles that I've seen before include "precarious situation" , "Impending threat" , "imminent threat" , "Imminently impending peril, disaster" , "It was perilously poised overhead" .)