Priscilla, known lovingly as "Gogo," which means "grandmother" in the language of her Kenyan village, attends Leaders Vision Preparatory School (LVPS) and is now in the 4th grade.
In addition, for the past 65 years she has served her village as a midwife, delivering countless numbers of babies, including some of the students she now attends class with.
What motivated Gogo, a great-great-grandmother, to attend school for the first time in her life?
"I'd like to be able to read the Bible; I also want to inspire children to get an education."
Gogo approaches unschooled children, strongly urging them to attend school. "They tell me they are too old," she told the BBC. "I tell them, 'Well I am at school and so should you.'"
"I see children who are lost, children who are without fathers, just going round and round, hopeless. I want to inspire them to go to school."
Ironically, LVPS at first rejected Gogo, until they realized how determined she truly is to receive an education.
"I'm very proud of her," Headmaster David Kinyanjui told the BBC. "Gogo has been a blessing to this school, she has been a motivator to all the pupils.
"She is loved by every pupil, they all want to learn and play with her."
Gogo participates in all of the classes, not just English, as she studies math, drama, dance and singing and takes a PE class.
In her blue uniform and red tie, Gogo also tells her young classmates stories of the past, so that the village lore and customs can be passed on through them.
Being 90, where does Gogo stand in the record books for aged students?
According to the BBC, she is the oldest primary school student ever, the prior record according to the Guinness Book of Records, is held by another Kenyan, Kimani Maruge, who attended primary school at the age of 84 in 2004, and passed away five years later.
Gogo's message is this: "I want to say to the children of the world, especially girls, that education will be your wealth ...
"With education you can be whatever you want, a doctor, lawyer or a pilot."