That message means that no inputs are allowed into the tool other than those that were provided to the original tool.
If you student mean for you to be able to vary the inputs and outputs, then there should be a P on those parameters on the model diagram.... Get them to open up the model and right click on the appropriate ovals/bubbles/whatever and select the '... make variable from parameter ….' (or words to that effect).
So it is simple... no P, no input variables or output results