Before the pandemic, the school in Effingham Lane, announced it would take pupil;s through to their GCSES, with the first cohort taking the exams in 2026.
But after reassessing their strategy, they have taken away the provision, which children now having to find alternative schools for their GCSEs.
One parent, who did not wish to be named as their child still goes to the school, called the Crawley Observer with their concerns.
They said: "We chose the school because of how good it was and how it would take our child through to GCSEs. But now we have to go through the stress of finding another school for our child after Year 8.
"We didn't know anything about the change in advance. It's such a shame because it is a lovely school and our child is so happy there."
The school has said they are in close contact with parents and will help them with the alternative plans they will have to make.
A spokesman for Copthorne Preparatory School said: "Before Covid struck Copthorne announced plans to take pupils through to 16, with the first students due to take their GCSEs in 2026.
"Since that decision was taken the pandemic happened – a once-in-a century event that required Governing bodies of all schools to reassess their strategic priorities.
"After a comprehensive review, the school therefore decided to focus its resources instead on a substantial programme of investment in its facilities and infrastructure – something that will cement its position as the leading prep school in the area.
"While this was undoubtedly the right thing to do it was not a decision that was taken lightly, and we recognise that it will disappoint those parents and pupils who had wanted to stay on at the School.
"We are in close touch with them and will do whatever we can to help them make alternative plans for their children’s education."