Pupils and staff dressed up for the event which was the first of its 50th anniversary celebrations.
Pupils and staff dressed in costumes from movie Grease and post-war Britain while the canteen provided an American diner-style lunch.
The school opened on September 9, 1958, with 247 pupils. Five headte-achers later it now has almost 1,400 on roll and a workforce of more than 200.
During the morning Year 7 pupils learned about the school's history and then released more than 230 yellow and blue balloons, representing the school's colours.
A commemorative service was held at Deeping St James Priory Church for 300 people.
Head Chris Beckett said: "The whole day was a fantastic success and a terrific way of launching our celebrations."
The school is holding an open day from 9.30am to 2.30pm on Friday next week. Visitors will be able to tour the school, see lessons and memorabilia.
The school is seeking memorabilia, stories and photographs relating to its first 50 years. Call 01778 342159 or e-mail office@deepingschool.org.uk