Is there any legal means to take action against a school that won't provide grades in a timely manner?

Power School is a common and accepted way to share grades with parents. Most homework is graded by a check mark, and if you want the formula that each teacher uses to grade, just ask them. (I used to manage similar systems in schools). There is indeed a value for the check, homework in total will be a percentage of the grade. (For example, if homework is half the grade, and you do all the homework, and all homework receives a check, this will be 50 points added to the other grades in some way that they will total 50 points if you got perfect scores in the tests and quizzes). The longer a year goes on, in any grading system, the less each test is worth compared to the running grade in improving a grade. It is simple. If your child is getting A's her grade is getting improved, no matter how long it takes a teacher to enter the scores in the gradebook. If she is failing, the grades will drop. You should keep following the grading system online, and if it is not being updated within a week or so (a normal time period which allows the teachers to grade everything by the end of a weekend), ask her, and if the answer is not satisfactory, ask the principal. The legal means is to ask how it works. I see no other thing that is being "done" to you to justify any legal means through the court.

You don't state the age of your child. I am assuming he/she must be in elementary school (my middle-school age children are very aware on their own of how they are doing in their classes and what areas need improvement). If you can't tell from Power School and you think there might be an issue, ask the teacher(s) for a conference so you can discuss what areas may need improvement and what you can do to assist. I don't really see this as a legal issue.