Possible Clues:

• It's hard to teach what society regards with distaste
• Unacceptable conduct
• Frowned-upon conduct
• Ill manners
• Lack of etiquette in rotten class
• Reprehensible conduct
• Unacceptable behaviour
• Clue from error in social ways
• Naughty pupils showing ill manners
• Unruly class's inappropriate behaviour