Is this a during school hours thing or after school thing?
I don't know if after school thing is necessarily wrong, as it in theory would be optional. It also really depends on what exactly is being studied, if the intent is really just to study literature then I'm okay with it. If the kids are being encouraged to shout praises to the "good Lord" then I would certainly be against that.
I would also say other religious texts and general literature should be involved.
It's really tough to walk the line though between reasonable and unacceptable once religion gets near a school.
Probably safer to just have a 100 foot restraining order.
As far as reporting this, I don't think there is a 1-800 number. You have a number of options, you can express your concerns with the people involved (hopefully in a way they will understand and won't actively resist you). Another route might be to discuss your concerns with the school officials, and then work your way up the ladder from them.
Beyond that, is to file a civil suit or injunction (something like that) with the local magistrate or a judge. You'll probably need evidence for that. Evidence might include if there was a pamphlet or flier handed out by the person(s) running the Bible Study or testing material that might be handed out. Of course the evidence would have to be clearly linked to the Bible Study. I'm not sure if pictures or video would be acceptable as evidence and that might be taking things kind of far and if you've already expressed your concerns, saying "hey can I take some pictures" might not work hehe.
Another option might be to write or call your local representative, either city, state or federal, again it depends on how concerned you are.
Thinking about this more, this seems odd. This country has churches for a reason, if you want to study the Bible, go to church. Why does studying the Bible need to be done at a public school? Did the local church burn down?