In argumentation, there are 4 types of claims. I suggest choosing a "policy" claim since you are advocating a change in future practice. Here are the 4 types of claims.
•fact: claims which focus on empirically verifiable phenomena
•judgment/value: claims involving opinions, attitudes, and subjective evaluations of things
•policy: claims advocating courses of action that should be undertaken
•definition/classification: indicates what criteria are being used to to define a term or what category something falls into
Also, there are two organizational models in policy debates: Cause/Effect and Problem/Solution. Pick either one for this topic, but I think problem/solution might be better if there is evidence to support the change. Then, organize the paper like you would most others. All writing is essentially persuasive.
A. Material to get the reader's attention (a "hook")
B. Introduce the problem or topic
C. Introduce your claim or thesis, perhaps with accompanying qualifiers that limit the scope of the argument.
Qualifiers, according to the Toumlin model of argumentation, are:
•"phrases showing what kind and degree of reliance is to be placed on the conclusion, given the arguments available to support them.” To set the degree of certainty, state a “qualification—‘usually,’ ‘possibly,’ ‘barring accidents,’ and so on.”
I would say that you can start off in one of two ways.
First, you could start by talking about what sorts of problems your solution would solve. In other words, you can start out by saying why it is bad to have grades. So you might talk about how grades reduce students' self-confidence or whatever it is that you think is the matter with them. (I assume you're talking about grades like A, B, C not 1st grade, 2nd grade).
Second, you could talk about why your solution would be good. In other words, you could paint a picture of what schools would be like if grades were abolished.
Both of these are similar ideas, I'd start off with one of them.
A good way to start is by giving an alternative that has worked. You can start by talking about a few progressive school that has done without grade and have produced great students. In fact, the president of Harvard University, Drew Faust, is a graduate of one of those colleges - Bryn Mawr College in PA. This school, as well as others, speak of education apart from grades and have created a culture were learning and critically thinking is the key. On the flip side, talk about other schools that have problems with false grade and grade inflation, which often happens, even in elite schools.
A pursuasive essay involves proving one side of an argument against another side. You will have to develope the best arguments to form an essay. When you are writing a persuasive essay you need to explain your reasons to agree with abolishing grades in school and all the reasons to disagree with abolishing grades in school. Then take all the reasons and figure out why your position on abolishing grades in school is better than the other positions. When you decide that, you can use this sentence to start your essay. You should only mention arguments that you can backup. You don’t mention arguments that many be questionable. By the end of the persuasive paper the reader should be convinced about whether it is good or bad to abolish grades in school.