The 25-year-old pop star personally emailed Jacques St Pierre, 17, after he organised a school assembly with an anti-bullying theme at the Etobicoke School of the Arts in Toronto.
He sent out dozens of emails to celebrities asking for their support and was shocked to get a personal response from the Marry The Night singer.
She said on the message: 'I just wanted to tell you how proud I am of you for being such a strong advocate of the LGBT community in your school. There should be more little monsters like you.'
St Pierre told a local TV show: 'I got called the gay kid, the fag, because I liked to be in the school plays. I lost my best friend because he joined in with the bullies. It's not fun, I've been there, I've been bullied. Before that, I didn't know bullying could affect people so severely.
'No questions asked, Lady Gaga sent us a video. I watched it, and I started crying. I'm a huge fan. It's kind of embarrassing because I love her so much. I couldn't believe it.'