If you are a high school teacher or university lecturer, what is the best way to conduct an online class so that students will get the most benefit out of it?

Zoom

Or they can pre record a lecture and send it to the students via email attachment or other social media.

Frankly I think this is better than being in the classroom because if the lecture is pre recorded and sent to the student, the student then has that lecture to listen to anytime they want as many times as they want.

As someone who has a short attention span, I can tell you that I wish that option was available to me when I was in HS.

Don't read the Idiot's Guide to online Teaching by Jeffrey Toobin.

It must be difficult for some subjects. I'm thinking of my maths and chemistry classes when at high school. In the maths class, the teacher would introduce a new concept, point us to some problems in the text book which we tried to solve individually. Then we would be asked to check each other's answers, which often caused very productive interaction and discussion among students. ("How did you get 4 as the answer? I got 3." "Because you forgot about .... you idiot." etc. ) Similar in chemistry. The teacher introduced the idea, then we got to do the experiment in groups, prompting a lot of interaction, and often concluding with the teacher asking, "Well, Jimmy's group have either disproved a fundamental law of physics or got their measurements wrong. What do you think?" I imagine it's not so easy to get that small group interaction online.

Make it simple and short.

 I think Line is more common here. After the class send a video to the students who can review the lesson and build a library.

I thought about that too as l always use line but Line videos are limited to 5 minutes. That's the problem.

Online course too expensive for such few hours, even the Open University of UK

It say whole year but count the hours properly. 2 hours, once a week, So 48 hours only per year. That is like 4 days a year. Then there are holidays. So it won't be 48, it might be around 30-35 hours only. Paying huge money for 3 days lesson, just for the certificate?

Why do you ask? High school and university teachers have to know that beforehand.