"You're still under the protection of adults."
And those adults are the teachers and probably parents. Not some guy she's interested in.
"I don't think it's good for you to fall in love with an adult who will so carelessly touch a high school student."
Good way of putting himself down
and treating her as immature at the same time. He's clearly not that careless, and she's not so immature he has to decide for her what's good for her.
"Think of the fans that you've betrayed."
Fans are secondary. Taking care of yourself is your primary concern, and that includes relationships. For the fans, it basically comes down to whether they think the idol deserves a boyfriend. If they don't, they don't care about the idol, and I wouldn't call them true fans.