If you’ve selected the International British (aka Cambridge) Curriculum, you’ll be able to start at any time of the year.
The curriculum is flexible and a self-paced learning platform (virtual school) and doesn’t run a traditional school year with fixed terms, scheduling, or lesson times. As such, the learner's school year starts on the day in which you enroll and will run for 12 months from that point.
Bear in mind, the courses are 30-34 weeks in length, and students can complete their level before the 12 months are up.
For the IGCSE, AS and A levels you’ll just need to keep the Cambridge June / November exam times in mind and ensure that the student has completed the course work by using the student tracker.
If you’ve selected the CAPS curriculum, you’ll be able to structure your child’s day-to-day learning in a way that suits you. You will however still need to follow the same exam schedule as the National Curriculum.