Efficient use of the class time when all lectures are already on Youtube

In the US, a class that counts for 3 credit hours normally meets for about 3 clock hours per week. It is often one hour at a time over the course of the week, though it might be otherwise. The typical student in such a course is expected to spend an additional six hours per week on the course, for nine hours total. I'll assume similar expectations for Israel, though I don't know.

You have an opportunity here to "flip" the classroom, though with a few caveats. To flip it, the students would spend their time as follows. About 3 hours per week watching the videos and taking notes. About 3 hours working independently in studying separately from the professor, and about 3 hours in a face to face situations.

A good use of the last segment, in a field like CS, is to have the students work together in pairs and small teams on exercises related to the lectures in the most recent video(s). If you use too much of that face to face time with questions then there will be little time for active work, so a better plan is to have a way, such as a mailing list by which students can ask their questions as they arise while watching the videos or afterwards. To make this as efficient as possible, the students are encouraged to answer each other's questions online and the professor need only enter the conversation when there are misconceptions expressed.

But, much of what would be "homework" is done with the professor (and TAs) present to offer advice to the teams. Teamwork (and pairing) is most effective here since the small groups can often answer their own questions as they arise so that the prof need not answer every such question or deal with every difficulty. The goal is that very few groups are truly "stuck" at any moment, which is normally not the case for students working singly.

So, an hour of watching videos, an hour in "class" working problems and the remaining hour is for other exercises or readings. This balance can give the required reinforcement and feedback that most students need in order to learn. The mailing list ties it all together and lets the students make progress at nearly any time of the day or night without overly burdening the professor.

Also note that with a mailing list, any question asked and answered is seen by every student. This alone is an advantage, especially when students "don't know what to ask" or are too shy to bring up what they see as their own failings. Other social media might work, but I found a simple mail server to be a good tool.

The main caveat is that a 50 minute video is a bit long for concentrated study. Better if they can be broken up into 10 minute segments. Even a recommended pause after 10 minutes or so for reflection is a better solution than a long video. Then, the student watches for a few minutes, perhaps formulates questions for the list or for later study, and then repeats. All of this prior to the face-time that will provide reinforcement of the ideas.

If the students work in groups in the face-time periods, you may need to define the groups or at least guarantee that they mix it up. This helps avoid slackers. If you need to "mark" or "grade" participation, then you also want to consider peer evaluation within the groups. But the "extra" hour (not watching videos and not face-time) can also be used for graded work if you need that.

Another caveat is that you need some assurance that none of your students will be much disadvantaged with the system. People with sight or hearing difficulties might have a different response than those with more normal facilities, depending on what accommodations can be made.

Since some students will not want to watch the videos, and they are not obligated to, you should teach the class as though the videos didn't exist.