CBSubs at this time can only manage 1 usergroup per plan under the Access tab. You can use CB Auto Actions to manage as many usergroups as you like using the below auto action configuration.

Plan Active
Global
Triggers: onCPayUserStateChange
User: Automatic
Access: Everybody
Conditions
1: [var3] Equal To PLAN_ID_HERE
2: [var2] Equal To A

Plan Expired
Global
Triggers: onCPayUserStateChange
User: Automatic
Access: Everybody
Conditions
1: [var3] Equal To PLAN_ID_HERE
2: [var2] Not Equal To A

You'd then use a Usergroup action which will let you add, remove, and even replace usergroups for a user.

It doesn't sound like you need to manage multiple usergroups per plan. The Course 1, Course 2, etc.. plans sound like they're just lifetime subscriptions which would give their corresponding usergroups of Course 1, Course 2, etc.. Classroom is the only usergroup you're actually revoking and having a plan that gives the Classroom usergroup under the Access tab should be enough for controlling it. They'll always have Registered since it's not controlled (well shouldn't be at least) by any plan and is the default registration usergroup. So my guess is your plan setup would be as follows.

Community (exclusive, free, lifetime)
Classroom (exclusive, paid)
Course 1 (non-exclusive, paid, lifetime)
Course 2 (non-exclusive, paid, lifetime)
Course 3 (non-exclusive, paid, lifetime)

They would be Community or Classroom with community just really being a means of registering without Classroom. I guess an alternative is the following.

Classroom (non-exclusive, paid)
Course 1 (non-exclusive, paid, lifetime, conditioned to display if Classroom is active/selected)
Course 2 (non-exclusive, paid, lifetime, conditioned to display if Classroom is active/selected)
Course 3 (non-exclusive, paid, lifetime, conditioned to display if Classroom is active/selected)

Regarding timed release configuration it sounds like you've it already configured properly, but that's something you'd need to test carefully. I'm not entirely sure that's how I'd do it however. I'd probably configure it as follows.

Joomla Categories
Classroom
- Course 1
- Course 2
- Course 3

Basically establish a Joomla article category per course. I would then create a plan that gives access to a course. Users would simply pay per course OR they could subscribe to all courses at once using the above suggested plan structure. You wouldn't need to worry about timed release substitutions. When something releases would just be the Start Publishing date when creating the article (e.g. if you want it accessible in 6 months then set the Start Publishing date 6 months from now).

Personally that'd be a lot easier to manage. No special usergroups even needed since CBSubs can directly control access to Joomla categories, but the usergroups could help with access controls elsewhere so maybe still a good idea. In short you'd create your articles and be done; there'd be no further management and no timed release substitutions to worry about. The timed release usage is useful in cases of "if they've been subscribed for 3 months then display XYZ".

I do not want the students to be able to choose which course to purchase and which course to skip, or in which order to take them.

That's not really an issue when you use conditions to limit what they can access. The courses being lifetime subscriptions means you can have them chain together. So for example under workflows Course 2 would be configured to be unavailable until Course 1 subscription is active. This means they have no choice but to go Course 1 > Course 2 > etc..

The timed release stuff is just a date comparison. There's really nothing fancy about it so whether you can do what you're wanting with it is based off that fact. For example you can compare against their subscription date, their expiration date, etc.. So if they've been subscribed for 3 months show XYZ course is absolutely possible. The status of their subscription is entirely separate. They can have a subscription date, but be expired. Some examples as follows.


The above accepts any subscription status for plan 4 except non-existing (which is a status of I). This usage is explained within the description of the cbpaidsubsbot within Extensions > Plugins if you'd like more details regarding what's available. The problem with doing it this way is you need to surround every article with it as needed so it's up to you if you want the extra work when creating lessons or courses.