When does Percy Jackson meet Chiron in The Lightning Thief?

Chiron is a centaur and the famous trainer of heroes (such as Hercules). In chapter 5, Percy wakes up in Camp Half Blood. Grover takes Percy to meet somebody named Chiron. However, this is not actually their first encounter. This is the first time Percy sees Chiron in his true form, but he realizes that Chiron is Mr. Brunner.
"Mr. Brunner" was Percy's favorite teacher, and we were first introduced to him in chapter 1 during the field trip. At Yancy Academy, Mr. Brunner taught Percy all about Ancient Greece and the history of the gods. Mr. Brunner used a motorized wheelchair to hide his true centaur form. We learn that "Mr. Brunner" was merely a front that Chiron assumed.
So although Percy first encounters Chiron in chapter 5, he has really known him for longer. He meets Mr. Brunner at Yancy Academy, and at Camp Half Blood, he learns that Mr. Brunner is Chiron.