I do not yet know if it will apply to all children adopted from public care, or those adopted after a certain date as per the admissions code. No cut-off dates have been specified in the information released so far. Happy to be corrected in this, but I can't find any.
As far as letting the school know, the information on the PAC website says:
The DfE has indicated that this will be made available to schools via the school census which will identify children adopted from care that have been declared by the parents at the time of admission or later.
So, if you are comfortable with doing so, I would let the school know now. Some schools are collecting data, whereas others may be waiting for parents to volunteer it.