Pupils, parents and staff witnessed the official opening of the new extensions at Glebe Primary School in Southwick.
The school, in Church Lane, is a third bigger, with an overall increase in capacity from 420 pupils to 630, and the extension has provided new classrooms and other facilities.
Head teacher Jo Kelly said: “We have a fantastic new building and we are very proud of it.
“A great thing is that we have the facilities now for all the children in the years to come.”
West Sussex County Council identified the need for the additional places to meet demand as a result of an increasing population in the Southwick catchment area.
The development is focused to the north of the school site, extending existing school buildings, with playing fields and parking to the south.
The five, single-storey, flat-roof extensions includes two new libraries, a reception classroom, three year-two classrooms, two year-three classrooms, a new hall and seven group rooms for use by each year group.
There are also new hard and soft play areas, improved car parking and better pedestrian access.
Shirley Osborne, who was a pupil at the school in the early 1960s, was invited to cut the ribbon at the ceremony last Wednesday.
She has worked at the school for 25 years, starting as a teaching assistant and now in the office.
Shirley said: “It was so different when I was a child, just a square where the hall is now. There are so many parents here who were pupils when I was first working here.”
The opening ceremony also included a dance performed by pupils, which Miss Kelly said was all their own work, and the cutting of a cake to share with everyone present.