The building plans for the $2.3-million stage 1b are out for tender. Building is due to start by late next month.

Middle school head Jason Przibilla said the new building would be attached to stage one a science wing with prep room and classroom.

He said stage 1b would include four general learning areas, an open-plan area in the middle, a seminar room and a teacher's office.

An outdoor learning area and tennis courts will be created. Landscaping works will also be completed as part of the second stage.

"It will give us the feel of an actual school," Mr Przibilla said.

The school will expand next year with its first-ever year eight class. Holy Trinity opened its middle school to year sevens this year.

It will include years seven and eight next year, and up to year nine by 2016.

Mr Przibilla said the school's 20 year sevens would form the inaugural year eight class.

He said about 35 students had registered interest for year seven next year.

"We've had quite a few of our existing year sixes interested," he said.

"We've picked up some from Dimboola and Nhill and the local primary schools.

"We're looking at two classes for year sevens next year."

Mr Przibilla said he was thrilled to field three middle school classes in 2015.

"It's really good," he said.

"We thought we might lose a couple of our students with siblings at other high schools, but that's not the case."

The extra classes will require two new staff members in full-time roles.

Mr Przibilla said those positions would likely be filled by primary school teachers.

He said the school might employ other staff on a part-time basis to work with both campuses.

Mr Przibilla said this year's year seven class would continue to develop leadership skills next year.

He said students would work with the new year sevens through a buddy system to introduce them to secondary school life.

"It's a really important year," he said. "We understand they will be the leaders, not just this year, but in year eight and nine.

"We value developing leadership."

Mr Przibilla welcomed the strong support the school had received from parents and the community.