I'm a bit surprised that "I am going to high school this year" is not accepted. I guess they mean "I" is to be a "new enrollee??"
Your English sentence sounds like you are already in high school. The e...ana tense indicates it's a future action. "I am going to go . . ." should have been acceptable.
Probably because komo means to enter. Aloha
"I'll be starting high school this year." (Reported 2020-08-06)