If your students are doing more of a video in which they need to have their script read entirely word for word, I would recommend using cueprompter. The other day Tony Vincent tweeted about cueprompter. Cueprompter allows your browser to work like a teleprompter. When students type or paste in their text, they can then select Start Prompter and a new tab will open that looks like a teleprompter. When ready students can select forward and then the speed they would like the teleprompter to scroll (can be adjusted during the presentation). If the user would like, the prompter can go full screen as well.
Once cueprompter is ready, students can go back to the tab that has Flipgrid and then start the recording. They can then open the cueprompter tab and start reading. When finished reading students can then go back to Flipgrid, stop the recording, and then trim the beginning or end of the video to their specifications.
Embedded below is a GIF followed by a 1 minute and 30 second YouTube tutorial.