I'm excited to hear from you! I would love to learn more about the girls and their family.
To answer your questions, I came across the girls during my research of the convent's school, located in York, as part of my Master's degree. Opened in 1686 to cater to the aristocracy, the school began to attract a growing number of middle class students from around the globe during the 18th and early 19th centuries, including your three relatives. They were, however, the only ones from Demarara (so I was very surprised to hear that there were more girls!). Most of the foreign students at the school were daughters of catholic merchants from the West Indies, Spain, Portugal, and France (and a dozen other locations), though the majority of the student body were British nationals.
My current research while focusing on the school will eventually also encompass the networks established by merchants and industrialists who sent their daughters to the school in the 18th-19th centuries. So, long story short, I'm trying to reconsitute as many of the foreign families as I can so I can track their family and business connections. Unfortunately, I've been hindered by the fact that, in most cases, the school's account books do not include the names or occupations of the girls' parents. I'm having to do a lot of chasing to make a little progress - hence my excitement in getting your post.
What a wonderful way to end 2007 - I look forward to hearing from you.