The best way to learn proper English is to read news report, and watch news on TV. Watching TV shows is a great way to learn casual English, slang words, understand culture reference and humor. If you have already watched these shows then you may recall the words used in the following dialogs.

...and my GRADE point average
fell from a 5.0 to a 1.8.
The Big Bang Theory Season 1, Episode 12

Mrs. Wunch in fourth GRADE.
The Big Bang Theory Season 9, Episode 23

Oh, tenth GRADE,
and he's just growing
like a weed.
Breaking Bad Season 2, Episode 7

...to be picked for a game in fifth GRADE,
I'm sure you do suck.
The Big Bang Theory Season 8, Episode 3