Fake It
"Can you help me find that person? I can't ask anyone for help and... you came to help me make a decision... can you please help me out...?" I asked her.
I know this might seem reckless to some but it's the best strategy I could think of at the moment in order to gain her trust.
This way she can't escape from me and later on, she might turn herself in and at that moment I can proceed with my plan that I had planned for her since earlier.
"W-What… did y-you...?" She was confused and taken aback at the same time.
Well, that's a normal reaction since I just asked the real culprit to help me find an imagery culprit.
"... Just think about it," I said, "I can use your help. You have quite some influence among the class and maybe outside too. I could really use your help in this one."
"N-no… I can't…" she still denied.