Before you go to the principal and school board, talk to the teacher and find out the school policies under which the teacher works. Our school has a policy concerning homework and how much and how often we are supposed to send home, so if the teacher is following policy then trying to get her/him in trouble for that is pretty much going to get you nowhere. I know a lot of people on here do not believe in following the chain of command at a school, but the first step is always talk to the teacher first.
Explain your concerns about the amount of homework, and tell the teacher how long it takes every night to do it all and ask for adaptive homework assignments. I don't like to assign homework unless it is strictly practice because you can't take an honest grade from homework, there are too many family members who do it for the student and I also believe that family time is more important.
If the math homework is something like this for example: 20 addition problems 2 + 2, 3+3, etc, ask if you can cut it into fourths and do five problems of each type instead of 20. If you see your daughter struggling to do it though that means she needs practice at it and no there is not usually time for a lot of practice in one subject in the classroom because there are other subjects that must be taught. Practicing letters is very important, do it first and get it out of the way. One good way to practice her writing is to have her copy her favorite story book.
Most students with SPED IEP's have written into them guidelines concerning adaptive assignments in class but generally they do not limit homework because many SPED students will need more practice at home to make progress.
My biggest concern is holding her back, don't. In our district the parent can refuse to have a child held back one time and they have to sign a form stating that they have used their one opt-out. This means the child can be held back later without the parents permission. Holding a child back in K - 2 is useless. That means when they get up to 5th or 6th and are really struggling we can't hold them back when they really need it. So if you can prevent that from happening please don't let them hold her back at that young age.
If you can't get the homework issue resolved at the school level, teacher then principal, then go to the next level, but don't try and jump the chain because believe me; nothing will cause ill-will faster than a parent who thinks they can force an issue by going around the teacher without speaking to them first. I had a parent come in two days ago to complain about something I said in class 2 months ago. She never called me, never attempted to talk to me, and didn't even remember exactly what I said but she called the superintendent first, who asked if she'd spoken with the principal yet,, of course she hadn't so she was told to talk to the principal and she was asked at that level how I had responded to her and of course she hadn't spoken with me. So instead of calling me right away and solving the problem she took the round about way, a lot of wasted time, and then was unhappy because I had her daughter removed from my class because I'm not willing to go through this for the rest of the year every time she decides to jump the chain of command.