A survey of more than 600 children with type-1 diabetes found that 46 per cent of primary school pupils with the condition said their parents had to quit or work less to help them with insulin jabs.
A total of 29 per cent of secondary school pupils said their parents quit or worked less to support them.
Parents feel the need to administer jabs themselves because of concerns schools are unable to help children avoid incorrect doses or missed injections.
A total of 65 per cent of primary school pupils and 87 per cent of secondary school pupils say school staff had insufficient training to help.
Diabetes UK chief executive Douglas Smallwood said: 'It is completely unacceptable that parents of children with diabetes are forced to forfeit their careers and risk financial hardship because of medical failings in schools.'
Around 20,000 children in the UK aged under 15 have type-1 diabetes.