If the teacher has a higher resolution than the students during a demo, or the student being showcased during a showcase student has a higher resolution than the students, the text on the screen is unreadable.
There are two ways you can remedy this:
1) Stop the demo, then lower the resolution on the higher resolution computer to an equivalent resolution or lower and then restart the demo.
2) Use the windowed demo and turn off the fit-to-window option on the student computers.
This will restore the ability to read the text on the screen.