When replying, you want to be nice to your English teacher (as she determines your grade), but yet still get the point across. If it was me, I would say something along the lines of...
" I know I am capable of much better work, this assignment was just harder for me and I couldn't comprehend it as well. I know this is no excuse and I will try harder and put in more effort next time. Yes, a kindergartner probably had better grammar in this assignment, but definitely not in all.
Hope this helped :)
