You're free to decide
As an academy, it's up to your trust to determine the length of the school year.
This is set out in the DfE's model funding agreement (see page 14). However, check your own funding agreement in case it's different.
Take staff contracts into account
Before making changes to the school year, particularly plans to make it longer, consider staff contracts and the effect this might have on their pay.
Teachers employed under the STPCD
If any of your teachers are employed under the School Teachers' Pay and Conditions Document (STPCD), then they must be available:
This is set out in paragraph 51.2 of the STPCD. Find out more about whether the STPCD might apply to staff in your school, for example if their contracts transferred when your school converted