After being attacked, Wes convinces Dalio they need to take a different route back to campus. What is ironic about the route Wes leads him on?

Wes says it's ironic because he had run through the woods looking for safety before. The irony is that, at that time, he was trying to get away from campus and now he's just trying to get back to campus.
After Wes and Dalio are attacked, they realize that they need a different way back to campus. They go through darkened streets and front yards until they reach the "wooded landscape" that Wes describes as a field that leads back to school. They run through it to get back and keep away from the people who almost ran them over and who threw something at Wes, breaking his tooth.
Once upon a time, Wes had run through the same woods to try to get to the train station. He wanted to get away from school; he ran away four times in four days. Now, however, school is a desirable place to be. It offers safety and security. To Wes, that is the irony of the route.