NOT A TEACHER
Hello, Tufguy:
The teachers have already given you the answer. I only wanted to add a few thoughts.
1. If you are writing for Americans, it might be better to stick with the word "class." The word "session" is often used to refer to a particular period of academic time, e.g., "I will be attending summer session this year. I plan to take two classes." The word "session" could also be used to refer to when the class is being given: "I wanted to attend the morning session of Algebra 2a, but it was all filled up. So I have to attend the afternoon session [of Algebra 2a]."
2. I think that one sentence would be much smoother and maybe more effective: "This class is very informative, so it's worth attending." / "It's worth attending this class because it's very informative."
3. Sometimes people write two sentences because they want to be more emphatic ("dramatic"): "This class is boring. It's not worth attending."