The speaker says she loves no one now, but she undercuts herself right away with "At least I think I do." That hesitation sets the tone for the whole poem. She is trying to convince herself that a crush has faded, while the details show it hasn't. Her heart no longer stops when he says "Hey," yet at a social that was "loads of fun," she still can't stop staring at him in his plaid.
The sting comes from being unseen. He doesn't notice her, and he ignores her shy "Hi" in the hall. Her claim that she doesn't like or love him "anymore" starts to sound like self-protection, a way of softening the hurt.
The mood is quiet, wistful and a little deflated. She imagines he might finally notice her when the year is over, then immediately says, "But I doubt it." The plain, simple language makes the disappointment feel honest and familiar.