What happened for her to get amnesia?
How'd the kid get hit? You push him/her in front of a bus? Your guilty ( and liable - look up your new favorite song, "Workin in a Coal Mine" ), but if the kid has amnesia he/she will not be able to testify against you. Any other witnesses? What do you have on them ( for blackmail )? Tell them to remember, "Silence is Golden". Was the kid hit by a piece of falling space junk? The Gov. did it!!!! Was the kid hit by a tree ( falling ), a kamakizi bird, or a meterorite? An act of GOD, your not liable, say "al-leh-lu-yah, and pass the plate", you has been fo-given. Praise the Lord. As to 'Slander', unless you can 'prove' ( beyond a reasonable doubt ) that the person is "knowingly" spreading lies about you, there is none ( no slander ). Proving it can be ( and is ) extremely difficult.
Amnesia is not caused by a blow to the head. It's caused usually by a stroke or a degenerative brain disease, Extreme psychological trauma, or electro convulsive therapy. It takes SEVERE brain trauma to cause memory loss and even then it develops over time. It's not like you slip and bang your head and then wake up with amnesia. You'd need a hatchet tot he head for that to happen. Chances are she's experiencing memory loss, not amnesia.
Add the fact to that your story has more holes in it than a screen door and I smile, nod, and bow out of this crock of sh*t.
Go to your school's Union...it sounds like there's no one else who can help you. Short of Legal action, though, you may be up the creek
Sue everybody. Slander is serious business. You need to keep records. Talk to your lawyer about it.