You have done exactly what we recommend to anyone who is being bullied and that is to report it to someone you can trust and who is in a position to help.
Is the social worker part of your school administration? If this is happening mostly in school, teachers, administrators or a school counselor should be helping you through this.
Do your parents know this is happening, are they supportive? If you haven't talked to them, let them know what is going on and how you are feeling.
How do these girls know that you are an atheist? Religious beliefs can be a very sensitive subject. If talking about your beliefs (religious, political, etc.) is important to you, you will need to be prepared for others to disagree.
There are a number of things that you can do to curtail the bullying.
1. Ask the social worker for suggestions that could help you avoid the bullying while it occurrs.
2. Come up with a response that you can use when they make the comments. Make sure it isn't something that would give them another thing to tease you about though.
3. Another suggestion is to use self-talk. When comments are made to you, or you are being left out, have something you say to yourself that helps you not to react to the teasing comments or being ignored.