In Hard Times, what is the main message that Gradgrind wants to communicate to the students?

The assertion that facts and hard data should form the basis of living is the main message that Gradgrind communicates to his students.  

Gradgrind believes that provable facts must underscore modern reality.  He communicates the importance of “two and two are four, and nothing over" to his students.  He believes any course of study that moves away from facts is not worthy of investigation:

Now, what I want is, Facts. Teach these boys and girls nothing but Facts... Plant nothing else, and root out everything else. You can only form the minds of reasoning animals upon Facts: nothing else will ever be of any service to them. This is the principle on which I bring up my own children, and this is the principle on which I bring up these children.

Gradgrind communicates his emphasis through the lessons he teaches his students.  He steers away from artistic explorations, emotional experiences, and anything that cannot be quantified.  The way he teaches his students is through valorizing fact-based reality.  

Gradgrind enhances this message in the way he raises his children.  Gradgrind says that "nothing else" should be evident in a child's mind.  This helps to communicate his primary message that nothing should supersede factual analysis in the way people view the world and their place in it.