I'm wondering what you mean by external exam marking. Does that mean that you don't make up or grade the final test? If so, that does put you in a tough situation.

In grades 11 and 12, I had little interest in history. I didn't even watch the news to follow current affairs. Now I mostly watch news commentary programs, and I read some world history every day. Getting teenagers interested in history can be quite a challenge. I think you are too hard on yourself, in thinking that you should have made it more interesting to them. Those "external" requirements kind of dictate what you must teach, if I understand the reference.

I see a silver lining in this situation. The 5 students who return for year 12 history will be the more motivated ones, who also likely to have more interest in history. That means you can teach at a higher level to them. You might be able to make a small class like that really some fun. You can give each student more attention. Each of them will have more time to participate in class discussions. You can make believe you're working in an elite prep school, where student to teacher ratios are intentionally kept low.