Translation:There is a lot of grass outside the school.
"tons of" - ha! Who would translate it that way? I'm glad they accept this now.
Your suggestion does have the correct subject verb agreement, but its not an accurate translation.
A better translation would be "There is a ton of grass outside the school"
Grass is uncountable so we use is and not are. Other uncountable words would be water, we don't say there are tons of water do we?
It should not be tons. The chinese translates as a lot. There is a word for ton in chinese.
A ton, or tons of grass outside the school sounds like there are piles of grass...