Some schools have shortened their lessons by so much that they are now breaching Education Department rules, which say students must receive 25 hours of face-to-face instruction every week.

Northcote High School was faced with a tough decision two years ago – it either had to wind back the VCE subjects it offered, or scale back its tutorials.

It choose the latter, and its students now spend 31 minutes less time in class every week than the Department says they should.

The Department has granted the school an exemption to its rules.

It is understood this has saved the school up to $100,000 a year in teaching costs, with many staff now required to teach an extra period a week.

More principals are being forced to make tough decisions like this as they grapple with inadequate funding, with one in four Victorian state schools now in deficit.

While it's unlikely that students will complain about spending less time in class, the Australian Education Union has raised concerns about the practice.

AEU Victorian branch president Meredith Peace said she suspected many more schools were breaching the rules.

"I don't think it uncommon," she said.

"Schools don't have adequate resources. They scrimp and save wherever they can. They are doing this because they are trying to keep their curriculum as broad as possible."

She said reducing class times created extra work for teachers, because it often meant they had to teach more periods. This meant more time preparing for classes.

"Schools shouldn't be blamed for being in a position where they are making these decision because they are inadequately funded," she said.

Parents Victoria executive officer Gail McHardy said schools should not have to make these decisions.

"Schools are pressured to deliver student outcomes, yet at the same time they are having to justify reducing contact hours," she said.

Northcote High School principal Kate Morris said the school's lesson structure was optimal for students.

"Northcote High delivers broad based curriculum programs to its diverse student community of which we are very proud. Student outcomes continue to improve year on year, as a result of excellent teaching, careful planning and resource allocation," she said.

She said the school ensured it used its finite resources to "improve student learning outcomes, enrich the student experience and support excellent teaching".

An Education Department spokeswoman said many schools had moved away from the historic system of six 50-minute periods to a structure which better suited their schools' needs. She did not respond to questions about how many exemptions the Department had granted to schools which breached its rules.

"Schools work with their communities to deliver a strong curriculum which best meets the needs of students, makes maximum use of resources provided to the school, and maximises the face to face teaching time that every teacher delivers."

A growing number of principals are struggling to balance their school budgets, with some schools hiring out their teachers to other schools to generate extra revenue.

Principals said they had been forced to cut programs and hire cheaper teachers due to inadequate funding.

Every Victorian state school student received $2253 less in state and federal government funding than the national average in 2013-14, making them the lowest funded in the country.