Go into the student record, then click on side tab Schedule. Then click on the subtab Workspace. Once in the below, you click the "Select" button under "Student Schedule (0% scheduled - Class credits: 0.0)":
It will bring up the below popup. From the "Platoon" dropdown, go ahead and select the platoon for the student. In the ES, they are based on homeroom teachers, so go ahead and pick their teacher. Let's say it's Aimee Fredette:
It'll bring up all the sections in the Fredette's platoon;
Select all of them, like below. Then click OK.:
They will appear under the Student Schedule section. All looks good? Click "Post", all the way on that bottom-right where the cursor is:
Once done, you can go to side tab Schedule, sub tab Schedule, and it now has the new platoon added.