That's because it's not action, but comedy and slice of life. you don't watch it for "tension" - though there can be other types of tension, such as bullying, dating, and other "psychological" issues. tension does not always have to be physical fighting :). People who like slice of life would love this, while people who like action anime like attack on titan and tokyo ghoul would probably be bored of this. Koro is cute, and also has a mysterious past, which keeps the viewer watching wanting to know just why he made that deal to teach in the first place, and who that mysterious woman whom he made the deal with, really is. There's some mysteries at the school too, like how the principal and higher ups seem to be "using" the low class students to motivate the others, and drama there as well. Koro is an uplifting character who cares about his students, and the show is written in such a way that the viewer also begins to care about the students. Defeating Koro is only one of many troubles the students face. Their main trouble is everyone thinking they are garbage, and trying to prove to the school, themselves, and their families, that they are actually all amazing. :)