Simply copy and past text into the text box after launching the app and watch it work. For the text and spelling options I tested it with, it worked great.
The extension is more advanced as it works within the documents that you use in Chrome to check the text you enter as you type. This might be something that is far too distracting for students learning to type and making frequent errors, but it also might be something that would help them better monitor text as it is entered rather than watching fingers on the keyboard. Only way to know is to give it a try. A video is inserted below that demonstrates the extension. Give this one a try. Please share your opinion as to how it works in the classroom setting.