The event, which takes place at the school this Saturday, October 11, will include a number of demonstrations, exhibitions and performances, as well as an address by headteacher Phil Birch.
Mr Birch said: "People can come and wander down memory lane and take a look at the displays and exhibitions. It will be an opportunity for people to come who haven't been back to school in recent times, as well as those who have, and is open to anyone with an interest in Garstang High School.
"Throughout the 50 years there have obviously been a lot of changes in society and in the world, and these are reflected in the school today.
"But I think the things that make the school rock and roll are the same as they always have been. The fundamentals stay the same and it is important not to forget that."
Garstang High School was first opened as a Secondary Modern School on October 21, 1958 by John Welch, a former chairman of Lancashire Education Committee and of No.2 Divisional Executive (Lancaster and Morecambe).
The building was dedicated by the Rural Dean of Garstang, the Rev AB Dex.
The school comprised a two-storey teaching block and was equipped to take 300 students. It was one of the first of its kind in the county. The cost was £99,500 - 256 new pupils, and 12 members of staff were admitted to the new school on September 9.
From 10am to 1pm on Saturday there will be an exhibition of memorabilia in the study centre. At 10.45am there will be an address by Mr Birch in the hall.
From 11am to 11.40am a music recital will be performed by the Garstang High School Concert Band, and from 10am to 12.30pm there will be a cooking demonstration in T1.
The event will also include tours of the school by students, video memoirs, gymnastics displays, entertainment by the dance group and choir, and refreshments provided by Friends of Garstang High School.
Mr Birch will also launch the appeal for a new sports hall at the school and there will be an exhibition of the plans for the building.
He added: "The open morning is the first of a number of events planned for our anniversary year and we would love for as many people to come as possible to make Saturday a very special day and celebrate the fact that Garstang has had a high school for 50 years - something which has been fantastic for the whole community."