This works like charm!
So I edited the code as per my requirement. And here are the changes: It will save the id number from the response into the environment variable.
var jsonData = JSON.parse(responseBody);
for (var i = 0; i < jsonData.data.length; i++)
{
var counter = jsonData.data[i];
postman.setEnvironmentVariable("schID", counter.id);
}