When copying and pasting text on a Chromebook Use Ctrl v to include formatting Use Ctrl Shift V for no formatting
Earlier I was working with students and they were copying (Ctrl c) and pasting (Ctrl v) text into a Google Slide. When they did that the formatting was included. In order for the formatting not to show up simply have them use Ctrl Shift v.
Embedded below is a GIF followed by a second YouTube tutorial.