Well this is certainly turning into a big challenge!
I thought that I would chase the other members of the foster family in the 1891 and 1901 census in case he was with his foster brothers but he seems not to be there either

Here are the details in case you want them:
1891 - 95 Portland Street, St Mary's, Newington, Walworth, London
Mary Griffin Head Wid 56yrs Needlewoman (shirt) b. Walworth, London
Alice Hooker Dau Wid 29yrs Charing b. Walworth, London
Alfred Griffin Son S 20yrs Bacon? ?? b. Walworth, London
Nellie Hooker Grandchild 2yrs b. Walworth, London
John Strudwick Friend Wid 57yrs Plumber b. Camberwell, London
1891 - 53 Cator Street, Camberwell, St Luke's, London
Walter W Griffin Head M 27yrs Labourer (iron foundry) b. Walworth, London
Elizabeth S Griffin Wife M 21yrs b. Walworth, London
So no Charles there...perhaps the Charles Griffin at the school is your man??
Marriages:
Walter William Griffin m. Elizabeth Sarah Wilson Sept Q 1889 St Saviour Southwark
Alfred John Griffin m. Amelia Hannah Strudwick Dec Q 1891 Kingston, Middlesex/Surrey
Alice Mary Griffin m. Harry Hooker Sept Q 1885 St Saviour Southwark
1901 - 43 Beaker? Road, St Lukes, Camberwell, London
Walter Griffin Head M 37yrs Bricklayer b. Newington, London
Elizabeth Griffin Wife M 31yrs b. Camberwell, London
Lillian Griffin Dau 9yrs b. Camberwell, London
Florence Griffin Dau 6yrs b. Camberwell, London
Benjamin Griffin Son 4yrs b. Camberwell, London
Clara Griffin Dau 2yrs b. Camberwell, London
George Griffin Son 1yr b. Camberwell, London
Frederick Wilson Brother-in-law S 29yrs Bricklayer b. Newington, London
1901 - 25 Ripley Street, Newington, St Andrew, Southwark
Alfred Griffin Head M 30yrs Coal Trolleyman b. Newington, London
Amelia Griffin Wife M 30yrs b. Newington, London
Ruth M Griffin Dau 8yrs b. Newington, London
Florence M Griffin Dau 4yrs b. City, London
John Strudwick Father-in-law Wid 66yrs Zinc Worker b. Camberwell, London
Not sure that that really helps much but at least it cancels out some possibilities

Jenny