The school has spent almost two years going through processes to have a middle school - years seven to nine - approved.

Principal Jeff Gork said the school, which is currently a primary school, completed feasibility studies early on.

He said it had since passed a series of Lutheran church committees and gained Lutheran, state and federal education approval to become a prep-nine school.

The school will cater for year seven students from next year.

Mr Gork said he was thrilled with the decision.

"We are all a little bit elated," he said.

"To get the final go-ahead is wonderful. It's good news."

Mr Gork said he was working with architects on building designs.

He said the project would go to tender soon and construction on the open-plan-style building should start in October.

Mr Gork said the new middle school would be built in two stages.

He said the first stage would be completed by the end of the year, while the second section would be completed in 2015.

Mr Gork said the school would introduce at least one year seven class next year.

He said it would include years seven and eight in 2015.

The school will become prep-nine by 2016.

Mr Gork said he was happy with parents' responses to the project.

"We've had a huge reaction. We have been inundated with expressions of interest for year seven next year," he said.

Mr Gork said the school planned to start with one year seven class of about 25 students, with preference to current students.

He said interest would determine whether the school included extra classes.

The school will also hire two new staff members for next year.

Mr Gork said one would be the head of the middle school, while the other would be a teacher.

He said it was exciting to see the development come together.

"It has taken a little bit longer than we thought it might," he said.

"It's exciting - we are genuinely excited about being a middle school."

Mr Gork said the school would work alongside the Victorian Registration and Qualifications Authority throughout the introduction of new year levels during the next few years.

He thanked people for their support throughout the process.

Year six students Tarlie McCartney, Ahren Koch and Abbey Hawker are all planning to be part of the school's inaugural year seven class.

They said they looked forward to starting a next level of schooling at Holy Trinity.

"It's a good school and there are lots of good subjects," Ahren said.