To get percentage of where student are at with Assessments, you need to use the On Track Report.
Go to Reporting > Assessment > On Track
For basic percentages choose the Standard On Track report and choose your Year Group and Checkpoint. Then click choose.
Once you click choose you will get 3 boxes for each Subject. Below, Expected and Exceeding, each one will have a percentage and number, which is the count and percentage of students working at each of these.
Click on the figures in here to expand the report and show you which students make up that figure.