What are my rights as a preschool when it comes to dealing with a very aggressive child in the classroom ?

You need to be careful, as you need to balance safety concerns for you and other children versus the rights the child has. You should document daily events and work carefully with administration about this. It is an administrative issue, and you don't want to be caught in the middle, and be accused of abusing the child in any way. If the administration isn't helpful you should consider resigning. Whether you are entitled to unemployment compensation if you resign will be at issue. You should consult with a lawyer who can perhaps write a letter to assist you.