Idioms and sayings help increase a child's ability to proccess information.

Go sit in on the class. The kid will hate you for it, but you will learn a lot of useful stuff. Talk to other parents.

Do your home work! That is the main part of most grades. Read the book or poem to your kid. This is a good way to help understand the book. It is harder to try and understand while reading. English is a very dum subject to learn. Especially for kids. The different ways we use the words confuses them so try to go over it with them when you talk. I didnt fully understand it until high shcool. Make sure you keep helping your kid and until he starts to move up to a position where he/she can do it and understand it better. Also many teachers will be happy to do a after school study hall where he/she can ask questions and get good expanations.

k12 online homeschooling-it works fine for me.

Work harder.

L

This depends TOTALLY on the child involved. Sadly our schools treat children is if they were clones of one another, and all too often, the child having difficulties gets abandoned as "one that will never amount to anthing" when the child may be gifted or bored (not having found something interesting to "read", etc. ). I personally didn't read beyond a 3rd grade level even into my sixth year of school, but then I discovered Sci-Fi and within a year, went from reading 3 levels below my average to 2 levels above (not to mention reading every sci-fi, fantasy, and "odd story" book in the school's library).

As for the English proper, that may be more difficult. Again, depending on the child and what he/she/they are having troubles with.   Some of us have difficulties despite how we "appear" to write or speak. 
I increased my vocabulary by leaps and bounds, using a trick one English teach taught me though:   she said:  when you come to a word that you don't know, if you can, try to figure out what it means by the context (not always possible though), and write it down.   If it is not essential to the story line to understand wait to look it up, so you don't break the concentration on reading.   Later, after you are done reading for the moment, look them up.  If you use them a few times in sentences with people, you will increase you vocabulary so fast people will count you as a prodigy :-)