I got the idea from the Demi Lovato song Heart attack.

The main character (whether it's an OC or a reader insert) has a quirk that allows them to emit light from their body, they can control how bright or dim the light is. Whenever their crush (presumably a 1A student) is near them, talks to them, etc., they blush, or in their case, their cheeks give off a very light glow; but not so much that it's unnoticeable. They try to hide it, so no one will ask about it and figure them out.