I echo what ButEmily says- this behaviour is consistent with insecure or disorganised attachment and possible neglect or abuse. Please raise her behaviour with the safeguarding officer in school. If you have already done so, there are some techniques you can try.
First of all, focus on keeping her in mind, and letting her know that you are thinking of her. Break any task she has down into smaller tasks, so she knows that she can check in with you frequently.
Ask her what she is going to do that evening, and then ask her about it first thing in the morning.
You could introduce two small identical objects, such as marbles, ribbons or kinder toys, which each of you keeps in your pocket throughout the day. Taking this object out and letting her see you doing so will let her know that you are thinking of her when you are not able to tell her. You could hold the object whilst talking to the whole class, for example.
This website has some good tips and background information.
I'd also recommend this book
I must stress though that these behaviours do raise some concern, so please ensure that the right people know about them.
Good luck- and well done for this. Many children are written off as 'needy', and it's great that you are taking the time to explore strategies for her.