I agree with NQC.
"Fing" instead of "thing" could just be the students' accent, and "we done" instead of "we did" could be their dialect.
For example, in my accent I would be likely to say "somethin'" rather than "something", but I would always write "something". I have been known to use "I done" rather than "I did" in speech but would never write it as I know the difference between dialect, and formal English.
If people speak in a certain way, it doesn't always follow that they will use the same words, or spell the words in the same way they are spoken, when using formal English.
If they did, then that would be disheartening.