More ways to express that:
Teaching is the easy part, correcting the homework is the (real/actual) challenge.
Teaching is the easy part, homework correction is the sweating one.
Teaching is the easy part, homework correction is the real chore.
As it seems, grading the homework can be an even still worse chore!
Homework anyone? https://teach4theheart.com/why-you-should-grade-homework-but-not-how-you-think/
Notice that saying that "teaching is the easy part" is a way of speaking, it doesn´t mean that is easy at all, but that homework correction is even more difficult than that. As if saying "if you think teaching is difficult, wait until you have to do the homework correction", sort of.