*hugs*
Hmm...
How much does the school know? They might know that you've made accusations and can't determine whether or not these accusations are true.
On the other hand, you shouldn't need to prove whether or not what you said happened actually did in order for the school to give cut you a bit of slack on compassionate grounds.
You should only have to prove that your ability to study was significantly impaired due to reasons beyond your control. Distress over the whole process could be considered a part of this.

I've got a bit of experience with this. Uhm. This year I came forward about abuse that occurred to me as a child. Although I have gone to the police about it, there isn't any evidence other than my word on that issue. On the other hand, I've just finished applying for a waiver for late penalties for my psych honours thesis on compassionate grounds. Even though there's no evidence that says I was abused, there is evidence that I was very distressed and impaired. That's the main thing, at least at my uni.

Have you mentioned the assault to your therapist at all?