"Teaching bad students is not fun."
Translation:나쁜 학생을 가르치는 것이 재미없어요.
것이 or 것은 is basically 것 with the relevant topic/subject marker depending on context. 건 Is a contraction of 것은. Have a look at this lesson by ttmik for a better explanation. https://talktomeinkorean.com/curriculum/level-2-korean-grammar/lessons/level-2-lesson-19/