I don't know how people are rallying around the idea that Jompa has secret feelings for this person, to be honest. Freud has been dead for about a century now, we're beyond his frivolous ideas. Generally, if someone abhors another with this type of passion, he/she isn't trying to repress an emotion. If Jompa really loved this girl, why would he be restraining his feelings for her? It's not logical and I think it's rather conspicuous that he's extremely distasteful of her.
I think you should just tell her that she's negatively affecting your life and causing you distress. And then proceed to telling her you would like her to stop being a nuisance (and don't try to sugarcoat things either. Don't be cruel, but just tell her how you feel without seeming harsh). And if she doesn't concede to your actions, you may have to take some sort of actions (whether it be informing the school principle, speaking to her parents, etc). Don't make up excuses for the text messages, be honest and stern. If she doesn't have a clue when you tell her things directly, she won't be any more dissuaded if you make excuses, I would imagine.