Developing a Thesis

At both GCSE and A Level, we ask students to develop a “thesis” to reach the top levels.

However, I’d be interested to hear people’s views on what a thesis actually is.

So far, I’ve come up with the following:

i) It is an over-arching argument that unites all of your smaller arguments.
ii) It is something you can agree or disagree with.
iii) It is particularly important to signpost it in the introduction, conclusion and topic sentences. I teach students to refer back to their thesis in each of these.

What am I missing?

Are there any good resources on what makes a good thesis?

And how do we teach students to develop a thesis, rather than simply explaining its general characteristics?