Child dependents of Ancestry Visa holders

I need some urgent advice. I was recently granted an ancestry visa and dependents' visas for my two children aged 15 and 17.

Both children are academically capable, and they have just started in a UK school. The 15 year old has adapted very well, but not so the elder child. The educational system is very different to that which we are used to in South Africa. The 17 year old would have entered her final year of school in SA this month, but here in the UK, since she has already missed out on so much work for A-Levels, would be able to only start the course in September this year and finish in June 2014. She would thus be 20 by the time she finishes school in the UK.

She is distressed by this. It would mean her leaving school at this point in the UK - she had been accepted by the school, but on the understanding that she would only properly begin work in September 2012, as the pupils in her year are already halfway through their coursework.

The options are either that she leaves school in the UK now and finds a job between now and September, when she will already be 18. She could then go back to school or start at a college to do her 2 year A-Level course. Or she could just go back to South Africa later this month to finish her final year of school there.

The latter option would be the most convenient, but before we make this big decision, I would like to know if anyone knows if the visa conditions for staying out of the UK for dependents are different to those for the main visa holder.

I am aware that as the main visa holder, I am permitted a total of 180 days out of the UK during the 5 years the visa is valid, and not more than 90 days at a time, if I want to eventually apply for ILR.

Do my children have to follow the same rules? I would be supporting my daughter solely for the entire time that she would be back in South Africa, and she would still be my dependent entirely. She would need to be back for the duration of her last year at school in SA, which would essentially be about 9 months until she had written her final exams. She would however be over 18 by then, which was why we had applied for the ancestry visa and dependents' visas when we did, while she is still 17.

I really need advice on this. If I can prove that although she was out of the UK for an extended period and that I was still supporting her financially and emotionally from the UK, would it still be possible for her to qualify for ILR after 5 years? I have had advice from teachers at the school where she is presently and they agree that she is in a state of distress as she is not coping with being thrown into a different system right at the end of her school career. She desperately wants to go on to study after school and is certainly capable of getting good results if she finishes school in SA.