How did you actually find out about this account in the first instance?

I think that two of the major problems you are going to hit with an ISP over this are:

1. There is no money in it for them so they couldn't care unless there is something blatantly illegal or against their policies going on. It is not as if this is a pay for service?

2. It is going to be very difficult for you to prove to them that you are who you say you are, unless you approach them through a law enforcement agency.............none of whom are likely to be that interested, given that no real "crime" seems to have been comitted at this stage, and there is no money involved. Your other recourse would be through a solicitor, but that will be expensive?

As liquid62 puts it so aptly, this could be a case of identity theft, it could be a case of your daughter sneaking around on the web.
Or it could be some sort of prank by one of her fellow schoolkids?