Hello! Students will receive that message stating “Your teacher didn’t expect you here…” if you have full units marked as hidden for their section. This is a separate setting from individual lessons within units as either hidden or viewable. You can un-hide units in full from https://studio.code.org/courses/csd and https://studio.code.org/courses/csp for example. For more info, see https://support.code.org/hc/en-us/articles/115001479372-Hiding-units-and-lessons-in-Code-org-courses.
Furthermore, while only 1 course can be assigned to a section, the course that is assigned to a section is just the first one that students will see when they log in. Students can continue to access any other course at any time by visiting studio.code.org/courses once they sign in, or just click on the Code.org logo in the top left corner, then click “Course Catalog” at the top of their homepage. Once students finish at least one puzzle in any course, that course will permanently “stick” to their homepage (studio.code.org/home) for easy access going forward. For example, if you have CSP Unit 5 assigned but want students to visit Unit 4, just have them type in studio.code.org/s/csp4 and complete just one level within that unit. It will then permanently appear on their homepage for easy access.
You as the teacher can change the course you’re viewing progress in in your ‘view progress’ tab by changing the dropdown list on the left side to any other course, to see how your students are doing in courses other than the one assigned. Changing the course assigned will just change what students see first when they log into your section. Hope this helps!