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She needs to be tested, and so do you.
So why not get yourself tested before you say anything. If you can say that you're negative on STDs, you're going to spare her some of the initial freakout.
If you're going to tell her, you owe her that much for your own self-respect.
And if you test bad, then she must know in case she's planning children.
Please don't do this again. Good luck in school.