student pay
If I set up a course account for free, can some students use it for free while other students pay for upgraded access?
YES. Just see the due date by which the students have to pay to be at the end of the course. Give them the opportunity to pay for an upgrade for the first few weeks (say), then just unclick the student pay checkbox, thus no longer requiring them to pay at all. In short, whether they have to pay or not is something you can change at any time, they have a grace period before being required to pay, and you can change any of these settings at any time.
You can pay a one-time fee of $14 (per course) to move your student project to a Member Hosting and enable Internet Access, as shown in the screenshots below. The teacher gives a deadline before which the course should be paid for by the student. This is visible below the top row when you open the project.
If the course already started and the deadline passed, you are required to pay before you can access files in the student project.
Note: if the student already has a subscription to CoCalc and has available upgrades, it is not necessary to pay the $14 fee; he/she can simply apply Member Hosting and Internet Access upgrades to the student project for the course in question.