The public education system in the United States is supposed to be the great social equalizer. It is supposed to give all students equal opportunity, and then those that do the best get the best prospects. It is not fair for some students to get unfair advantages just because they are labeled "at-risk".
I believe that requiring someone to participate by law will lead to resentment and perhaps the opposite effect that something like this wants to achieve, those parents who have a genuine desire to see their child succeed at school will be working with the school closely without a law being required.
I believe it would be impossible to quantify effort put forth by the teacher or the parent. It also creates another tax burden on the system to try and monitor the student's progression and the parents effort. It also may negatively impact the student's family who may not have the financial means of supporting the student as fully as the law would demand.
You can't force people to do things they don't want to. Making parents show up to deal with their kids' problems only gets you an uncooperative parent sitting across from you. Just let the school do its business, figure out the plan, and force the child and parent into it. Problem solved.
No, I do not think that schools and parents should be required by law to work together to improve at-risk students' educational prospects, because it is not the schools job to fix the parents. A school is not a general social service organization. The school needs to teach the children while they are there. After that it is the parent's problem.