Thank you both so much for your replies and yes, the following must have been where I read about the charity school being set up:-
Walter Thornbury, author of the 1878 book Old and New London, describes the Countess’ charitable work in glowing terms. Selina opened her house up for religious services and funded the building of dozens of chapels, as well as setting up the Spa Fields Charity School for local children. Thornbury claims that in 1780 the Gordon Rioters, who had swept through London destroying a large amount of property, spared the Spa Fields chapel when they were told of its link with the “good countess”.