The youngsters from the Old Park Road school even autographed the steel girder prior to a crane lifting it into place.
The capacity of the through-school is being increased from 1,250 pupils to 1,500 in years seven to 11 with effect from next September.
Building works began in June. The project includes a brand new building along with the conversion of an existing grassed pitch into a synthetic turf, which will be used by both the school and the local community.