Much would depend on what time period your great-grandmother taught in, the size of her school, and how wealthy the community was. But let's assume she was teaching in a small-ish, average school (20-30 children) in the early 20th century.

She likely taught two terms - spring/summer and fall/winter. Depending on how large the building was and the town population, she would have taught two groups (morning and afternoon) or one, with a break for lunch. The term could be as long as several months or as short as several weeks, depending on how long the teacher was available, how much the community could afford, and the population. Families were more likely to start their younger children in the summer so the curriculum included more arts and crafts, singing, and play than in winter. Older children were more likely to attend the winter session so they had a more liberal arts curriculum with history, science, literature, and math. Older children typically attended when they weren't needed at home and left when they were done; meaning they learned enough for their parents to feel they were educated, could no longer attend school because of other responsibilities, or wanted to pursue higher education.

However, she may not have taught both terms - men with teaching certificates would teach terms in between other jobs and could earn up to 4x more than your great-grandmother. She was also required to be single and likely lived with a local family if she wasn't from the area. Finally, if she was a typical teacher, she was probably 15 or 16 years old, just a year or two older than her oldest students.

The vast majority of the day was filled with recitations - children would memorize text from a primer or by listening to the teacher and then recite it back. These recitations would be performed at the end of the term for families and evaluators as evidence of their learning. They would have sat quietly practicing penmanship if the school had paper or enough slates to go around. If there weren't enough slates, they would have shared. Often, older children would help younger children after receiving instruction from the teacher but much of your great-grandmother's time would be spent on making sure the children were behaving and punishing them when they weren't. It was hard, boring work and teachers frequently exceeded their own knowledge and resources. Some children would bring books from home but even then, teachers had to spend a lot of their days listening to children recite the same passages over and over again.

So basically, there were different lessons and topics for children at different ages and skills levels but not nearly enough and most of your great-grandmother's time teaching was likely spent corralling children not much younger than her.

Source: Women's "True" Profession: Voices from the History of Teaching edited by Nancy Hoffman (2003) - especially "Teaching in the Little Red Schoolhouse" by Lucia B. Downing.

You might find what you're looking for in this text America's Country Schools by Gulliford.: https://www.amazon.com/Americas-Country-Schools-Andrew-Gulliford/dp/0870814222

I don't have a copy, but Curriculum: Foundations, Principles, and Issues by Ornstein and Hunkins reference it in the only reference I have to actual lessons in one room school houses. Orstein cites Gulliford that in the common schools of this era, teachers would have to prepare between 10 and 20 lessons per day.

This seems impossible, but also realize that the education of this era tended to be more rote memorization than in modern schools. You might be curious to browse this collection of 19th century school texts.

Why is this important? Well if you are a teacher working more out of primers and memorization methods, then it is more practical to tell a student, "work through pages 10 through 15 in your primer." Or, "practice your writing by copying these lines of text." There was also a fair amount of peer-tutoring practiced. Students were expected to help around the schoolhouse. In the winter, one student may have been tasked with feeding a fire, etc. Part of this would amount to older students being tasked with helping to teach younger students. If you combine peer-support with simpler, more primer-based lessons, it becomes more easy to see how one teacher could teach so many students of different levels simultaneously.