Tell her:
'Thy affection melts my cold, cold heart, oh sweet friend! How much brighter the sun shines with your unfaltering adulation keeping my soul erect.
And I do mean my soul, for there can be no joyous, fervently thrashing meeting of our maddened loins. Thine form, however pleasant to the eye and generally rather quite stunning in most respects, does not maketh my juices flow. Oh, I blame the caprice of nature, the randomness of environmental factors skewing my upbringing so. I must be forever apart from your fantabulous embraces as I am compelled to seek the companionship of other girls - neither prettier nor more charming than you, but just different in absolutely arbitrary ways.
What terrible fate, my kindred soul, to not be able to consummate this irritatingly one-sided attraction.
But pray, tell. Must this be the end of our relationship? Must we dwell on the impossible? Is awkwardness in each other's presence the only destination on the horizon? Can we ever stop thinking of the steamy, sweaty encounters that can never be? Is this the end?
I do still love you, in the boringly platonic, non-horizontally-inclined, carnally-challenged fashion. I know, it's a tall order to ask of you to stop getting all trembling in the leg at the glorious sight of my Apollonian physique, to stop your heaving breasts from bursting with passion, to cease the auto-lubrication reflex in-between your thighs.
But you, no, we must persevere, so that in ten, fifteen years time, when we're both older and on the brink of losing our youthful verve, when our presently elevated expectations will have plummeted something fierce, we might laugh at all of this, and perhaps have a short, guilty, dirty affair.
For now, my friend, Adieu. I shall meet you again when the fate ordains.
Probably tomorrow at school.'