If Student Leavers still have invoices being generated for them, then they may still have future instalments required to be processed against their account. To verify this, go to:

• Fees, Billing & Payments module
  
• Use the Search Fees, Billing & Payments field (top right) to lookup the Debtor (Parent)
  
• Select 'Instalments' (left menu) 
  
• Check against the future 'Instalment' to be Deleted
  
• Click 'Remove Selected Instalments (Preview)' (top right).