Are you just sharing information in the course or do they need to actually submit stuff?
If just sharing information, just open it up to the guest role but then do not actually add the students to the site. Then they will only have the guest role and not be able to get into other courses.
Alternatively, you could add an enrolment key to the courses that you don't want them to access and only give it out to those students that should be able to enrol.
Or you could possibly play with permissions - (Note, I have not tried this but worth a go maybe!!) - copy the student role and call it something relevant like "coursename student", then remove the View Courses without Participation permission. Then manually enroll them in the course you want them to be able to access. I think that maybe that will allow them to get to that course but not see any other courses but I am not totally sure!