With the Bologna process, teaching got a bit more structured than it was before: Students have to sign up for classes, they have to prove they learned something for each course, etc. Even before Bologna, there were occasions when students had to sign up: There were always obligatory seminars with too few places, so you had to put your name on a list (on paper, of course) or be there one hour or more before it started to make sure you could attend this seminar.
Today, we've gotten rid of paper lists and we now have electronic tools where students can "book" classes -- everything has become very service-oriented. However, there are different approaches on how and when to book a class. In Basel and Zurich, booking starts some weeks before the beginning of the semester and ends two or three weeks after the first lesson. It is thus possible for students to step back without any harm after attending a course for some weeks; but within four weeks into teaching, lecturers know who is a regular attendee and who will take part in the exam. So from week 4 on, you can plan actual group work.
In Konstanz, students can attend classes without prior booking -- they have to sign up for the respective ILIAS course, though. They have to sign up for the exam in the middle of the semester and can do so for two or three more weeks. As an instructor, you only get to know who is interested in actually doing the exam when the semester is almost over!
Students have a lot of time deciding if they like your class or not. Which is OK, but has a major drawback: I usually have some group work in my classes which are part of the exam. So if one of the group members decides to not to be interested in this class anymore, the group is shrinking or even reduced to one student. Which makes no sense and we then need to find other solutions for the left over group members.
What is even more annoying: Students often drop out without sending a note to their colleagues or to me. So for some weeks nobody knows whether someone is sick or has left the class.
Most universities also have introduced compulsory attendance, which is a bit strange when you consider that students are grown-ups with a certain interest in the subject. However, you have to count heads every lesson and make sure everyone who should be there actually is there. Students are allowed to skip up to two lessons (out of 14 or 15) without any negative consequences -- if they are seriously sick or have other duties (going to the WK [annual refresher course of the Swiss Army] or participating in an excursion) negotiations will start. However, I always tell students to drop me a short note when they cannot attend a lesson -- it may have an impact on what we do in class. Most of the students send me an e-mail or tell me in person if they cannot come the next week for whatever reason. So this works quite well.
However, some students don't tell you when they cannot show up for the next lesson and they also don't tell if they decided to drop out. In small institutes with only a few instructors and classes each semester, you always see those students again. And it's a bit awkward attending one class but having skipped the other one. At least I would feel a bit awkward, but when I ask students if they had been sick the other day in the other class, I sometimes hear: "Oh, I've decided to drop out, it's too much, I wanted to tell you, really! And, it's not about you, really!"
Maybe I have to change my attitude and see myself as a service provider, not a mentor: Attending a class or not is not about the subject or the instructor, it's just about balancing costs and benefits. And when there's no "contract," there are no commitments, you just leave.