With reference to Matilda I would say that, for whatever reason, she and George never actually married.
I would also say that the birth registered for Matilda Masey in Bham, Mar qtr 1867 is likely to be hers.
However, unless the records for Gem St school at the Library give some relevant information there will be no way to know for certain.
On the 1871 census there is an Eliza Mayes, aged 8, listed in the same place as Matilda. Could be a sister in a similar situation, unfortunately can't see anything else for her.