A Sydney mum has been left outraged after her daughter's lunch was criticised by her teacher, lashing out in a handwritten note.

The disgruntled parent wrote a not-so-subtle note for her child's teacher this morning, and sent her off to school with it. But not before she'd shared a snap of it on a local Facebook page.

"Unless you are going to 'pay' $$$ for my child's packed lunch and supply her with a drink, then please keep your opinions to yourself and stop telling my child what she can or can't have in her lunch box!" the note read.

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Users were keen to know what exactly what the teacher disapproved of, though most assumed it was the bottle of Apple Raspberry Cordial pictured (hence the arrows drawn beneath it).

In which case, some parents thought the teacher had a fair point. But the child's mum was quick to clarify.

The note had Facebook users divided; some parents were quick to defend the mother saying they could relate to having "fussy kids".

Several users also attempted to defend the teacher, arguing that she was just doing her job as healthy eating is also a "part of the curriculum."

Others accused the woman posting the note hoping for "validation," while some simply said it made her look like "a jerk."

The post comes ahead of World Teachers' Day tomorrow, an annual day dedicated to celebrating teachers and their contribution to our communities.

And while some parents insist teachers are there to teach - not to police lunch boxes - they are required to enforce the school's food policies.

Most schools have food policies to cater for children with allergies and to teach students about a healthy lifestyle with nutritious canteen options, nut-free zones, and nude-food lunch boxes.