For starters, look at this:
json structure: {"Class A", "Class B"}
Your JSON follows a wrong syntax! See:

Test your Jsons with Jsonlint!
A quick explanation:
{} (keys) for objects. Therefore, you need to enter the property:
{ "nome" : "LipESprY" }
[] (brackets) for vectors (list). Therefore, we do not declare the property:
["Valor 1", "Valor 2", "Valor 3"]
If we have more than one object, then we should put them inside a vector (list):
[{
"nome": "LipESprY"
}, {
"nome": "Larissa"
}]
Your JSON fixed, it looks like this:
["Turma A", "Turma B"]
Obviously you can improve your JSON a lot. But it is not the goal of my answer to "improve" what does not refer to the problem. I only mentioned this problem due to need to fix it.
Now to popular the data, can do so:
<!DOCTYPE html>
<html>
<head>
<title>Popular checkbox por LipESprY</title>
<!-- <script type="text/javascript" src="jquery-3.3.1.min.js"></script> -->
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
</head>
<body>
<label id="lb_checkbox">
</label>
</body>
<script type="text/javascript">
data = JSON.parse('["Turma A", "Turma B"]');
for (let valor of data) {
$("<input type='checkbox' value='"+valor+"'>"+valor+"<br>").appendTo('#lb_checkbox');
}
</script>
</html>
That’s what I was able to develop with the little code you posted, after a lot of effort! Now it’s up to you to adapt to your project...
Larissa, there’s little code there! You can’t run/simulate your code. Do you want to be popular with data coming via ajax or directly in the rendering you’re doing with PHP? Post the rest of the code...
– LipESprY
It is not recommended to use the comment to implement your question. To do so, edit your question and add the information you want to complement. Remember this information I mentioned.
– LipESprY
data comes in array form by ajax
– Larissa silva
What is the structure of this JSON? Where do you want to "fit" the checkboxes? Inside the
label? In your script it goes into the input. It’s wrong! See: appendTo() / Another thing, you’ve done one Tour? Another recommended reading: Inquiry - help center– LipESprY
Edit the question, heart!
– LipESprY