Possible Clues:

• Grade-schooler's woe
• Grownup nits
• Some ectoparasites
• Cause of some head scratching?
• How lousy to be frozen after half a century!
• The head may entertain such unwelcome visitors
• Vermin's headcount 95 per cent reduced, but still vermin
• They cause head-scratching?
• Wingless blood-suckers
• Worrisome subject of some school letters to parents