There is another reason this classification is bad. The types of groups should be consistent. However, even with good-looking and ugly, we have a problem. There are probably plenty of average-looking students in your class. It is very important that your groups include everyone in your class.
Classifications have to include all the possible classes to completely cover that topic.
Classification must include all of the topic. Classification must not mix the types of groups. Classification must not use categories that overlap. That will be different when you study about process essays. The thesis statement is similar to the topic sentence.