The character recently has been having problems at schools. He got called by the teachers and he however denies to them he has been having problems.
Then he talks to a friend and tells him this.
My understanding is akin to "If I get thought/If I become feed up with this, I only need to take a short break".
Then he explains he's used to not going to school or not having friends anyway.
This said, I don't see why he used だけだし here?