How to make a test invisible for students?

Teachers can prepare a test for students in advance. They don't want the students to gain access to the test in any way, of course.

Material visibility is one of the settings at the bottom left part of each card.

Green icon - the material/card can be seen also by students, blue icon - the material/card is visible only to the teacher.

If a teacher assigns the test to students, the test is made visible to students automatically. I.e. it is not necessary (nor advisable) to change the test visibility.

How do the students access materials prepared for them by the teachers?