Why is elementary school success an indicator of graduation rates?

Why is this sentence not a thesis statement?

It is not a debatable topic.

It is a question.

It is an opinion.

It is not a fact.

A thesis statement has a form of a seen sentence, for example

Watering flowers makes them grow faster

This statement can then be checked (proven or disproven) in an experiment (it's the hypothesis) or argument in an essay. It cannot be a question so the second option is correct