Hi,
I think you've gotten great advice on how to best document and lay out your case. Education is one of the most important things during childhood, so I wouldn't let the naysayers convince you to avoid trying.
Someone made a suggestion to see if the records show homework assignment completions.
If there are such records, you wouldn't even NEED a teacher's testimony. You can simply show the records as evidence, and demonstrate that homework completion was nearly 100% on mornings after they spent time with you, and far lower on other mornings.
I was once in court awaiting a hearing for my own custody case, and one of the preceding hearings was a mother who had lost custody to the father 10 months prior. She had lost custody because father moved 50 miles away, and so the court had to decide which one of them should have the child during the week. The court picked dad (I'm not sure why).
At issue in this hearing was that the kid (I forget exact age) was getting Ds and Fs. In mother's care, the kid was getting Cs and Bs.
The judge was VERY concerned about the poor academic performance. Grilled dad on why it was happening, what was he doing about it, etc.
The dad talked about how he tries to help with homework, how he's started looking into tutors, etc. Tried to say that it was just a child getting settled into his new routine, etc.
The mother kept arguing that in her care, the boy had done much better in school.
The judge finally said, "Obviously, this arrangement isn't working. I'm not sure why, but it's not working. This wasn't a problem before he was living with dad during the week. We need to change it."
Judge right then and there reversed custody back to mom. Boom, done in less than 5 minutes.
True story.
Let that be your hope. :)