Minto is where Claypoole School is located. This is indicated multiple times in the text. Examples include the heading of the letter Gemma attempts to send Mr. Donaldson which reads "Claypoole School/Minto/The Borders" and when Ross finds Gemma trying to find Claypoole on a classroom map and remarks, "Here's Hawick, and Denholm. Minto is too small" (Livesey 71, 91).

It's worth noting this quote implies that the village of Minto is too small to show up on a map of Scotland in 1959, and even today is incredibly tiny. Wikipedia lists the population from the 2011 census as 322 people. 

This lends a useful context to not only the town but also to Gemma's plight in the novel. She's bound to the school all the time and is unable to leave even on the weekends. Even if she was allowed to leave, the town is small which compounds the isolation and vulnerability she experiences as a young girl at the mercy of Miss and Mrs. Harris. Gemma knows it would be foolish to try to leave following the incident with Mr. Donaldson's letter. If she ran away like Ross attempted to before her, she has very little money in a small village with limited transportation options. This geographic context paints a very destitute picture for Gemma as she longs to leave Claypoole. 

Livesey, Margot. The Flight of Gemma Hardy: A Novel. HarperPerennial, 2013.

“Minto, Scottish Borders.” Wikipedia, Wikimedia Foundation, 6 Sept. 2019, en.wikipedia.org/wiki/Minto,_Scottish_Borders