I'm breaking up with this girl?

[MM]

I don't think that's a good way to start. Because here's how she'll likely respond: "Don't I get to decide what I deserve? Because I don't mind having to come second to schoolwork. Honestly, I feel the same way myself. And we can work together on that."

If you want out of the relationship, period, then you don't go beyond "I'm sorry; it's not working out." Offering reasons sounds to the person who doesn't want to end the relationship like you're leaving the door open to negotiation, and if that's not the case, that just makes things super-awkward and raises the likelihood you're going to have to share stuff you don't want to share.

All that said, if the main stumbling block here is the Facebook messages, then just adjust your privacy settings and tell the people you actually want to talk to that you know they're trying to be supportive, but you'd rather they just treat you like any other couple instead of making a big deal out of it. And maybe go talk to your school's counseling center, because bystanders really should not be having this much of an impact on your willingness to date.