Denial and threatening with a knife

My daughter is 14. Her autism was recognised a year ago and initially she was very accepting and quite relieved to understand herself better.

She has a long history of school avoidance and is currently not in school at all. She is in Year 10 and missed 9 months of Year 9. We are trying to get her to consider non school alternatives like online school so she gets a few GCSEs to help her in the future. However she flatly refuses to discuss any alternatives and keeps herself in a state of denial by watching TV and social media all day (nothing dangerous - mainly Taylor Swift and make up videos).

She says she is just dropping out and doesn’t want to do anything. Will not talk to me or to anyone. We have screen controls in place and she is getting increasingly frantic about them.  I let her have a lot of screen time to regulate herself but it is also her way of avoiding reality.  

This morning when I turned off her TV and her phone was off she had an extreme reaction. She got out a kitchen knife and was waving it around screaming about hating me and killing herself. It was all a big drama but frightening. I calmed her down but I am very shaken.

I don’t really believe she is suicidal she is just being extreme about getting what she wants as a teenager. But what on earth do I do next? I often back off with the view that she will find her own path and doing her GCSEs when she is older is fine I put her well-being first. But letting her ‘drop out’ does not feel like putting her well-being first. It feels like letting her down.