On a basic level, the answer depends on the specific student and teacher.
Really, the student-teacher relationship is meant to be challenging. Learning is never really easy, although it can be fun. Sure, facts can be learned quickly, but to understand the internal mechanisms of different processes and beliefs requires much more than a cursory perspective.
Students struggle to grasp deep ideas from a young age, which is challenging.
Teachers struggle to maintain and deepen their own understanding, AND they must also share and express these knowledges with grips of students who may or may not care.
I would say it is more difficult to be a teacher (from my experiences as a substitute teacher and tutor), although being a student can also be very difficult.
For students who don't care, school is only hard in that they must find ways to eke through without learning anything.