One tactic for aggressive behaviour is the shadow treatment. What I mean is; where she is, you are, right beside her. Or she stays right beside you. Continue to explain how we play properly (Gentle hands, good friendship skills, etc). As soon as she starts to react inappropriately, you pick her up (firmly, not all cuddly and snuggly) and put her in to the time out place. I know you feel like time outs are not working, but what I am suggesting is stopping her before she even gets to the point of hitting. You don't have to play with her every minute, in fact, you shouldn't. But within an arms reach so that you can nip the behaviour in the bud IMMEDIATELY. It is exhausting, for a period of time. But she will catch on eventually. If not you may need to let her go, as it is so unfair to the other kids who are being hit all day long.