I need to develop a thesis about the theme that "our actions can have unexpected consequences" for my Grade 11 English class.
The main issue you will need to address in developing a thesis is narrowing your topic and thinking of evidence to support your point. As it stands, the theme is rather generic and obvious. Given that we lack perfect foreknowledge, and thus do not know the consequences of our actions, our actions are always likely to have consequences we cannot anticipate.
One possible way to approach the topic would be as a set of cautionary tales concerning minor infractions. There have been a series of recent advertisements showing the unexpected consequences of texting and driving. While the driver's intention may just be to send a text, the consequences can be many dead or injured people. Thus you might think about focusing your thesis around the unexpected consequences of "just this once" or "I can handle it," arguing that people should not be surprised when statistical trends apply to them.
A more philosophical approach would be an attack on consequentialism. In other words, you could argue that consequentialism, as a moral or political belief, depends on our ability to predict outcomes, but if acts have unexpected consequences, that we can not base our moral judgments, or even life choices, on outcomes, but need instead to take a deontological position that we must do our duty and follow certain moral laws regardless of potential outcomes.