If you are working with children or vulnerable adults on frequent or intense basis then you are required to have a CRB checks carried out. However schools are an exception, this is because under DCFS published guidance which came into effect on January 1st 2007 (Safeguarding children and safer recruitment in Education) it is the head teachers or LEA decision of whether parent helpers will require checks or not. It is also stated within this guidance that it is no necessary in all situations to have checks carried out on parents when known to the school. For example a parent helping out on one open evening would not necessarily need a disclosure, because a list99 check would suffice, however a parent staying overnight on a school trip would need a disclosure.
CRB checks can be passed from one school to another as long as they are recent (less than a year old) and the school carries out appropriate checks with the other school. Schools and school teaching recruitment agencies are the only exception to this rule though and therefore if you have had a check elsewhere then the school cannot accept it. Schools are also the only organisation entitled to request to see a disclosure certificate from another employer .i.e. a recruitment companies providing temporary staff, ground maintenance company etc.
If all of your different schools are giving different advice then I would recommend to them that they look at the guidance Safeguarding children and safer recruitment in Education which can be found on the DCFS website.
Hope this helps.