Dell Vostro 3550 i7

Writing from a charity in the UK. We were donated a number of Vostro 3550 laptops with the proviso that we factory reset them. We don't have anyone working here who is able to do this and have already wiped 2 of them attempting. The business that donated them to us has not been any help having washed their hands of them. The security is locked and we cannot do a conventional reset. Is there any advice? To be honest it has probably cost us more in time/resource to date and if we can resolve this we probably end up donating them on ourselves.