While not explicitly stated in the novel, it is strongly implied that Miss Poole is the one who anonymously funds the school. The clues are:
* Her connection to Mr. Rochester: Miss Poole is Mr. Rochester's housekeeper and is clearly trusted by him.
* Her financial independence: She is described as "comfortable" and having her own private income.
* Her secretive nature: She's deliberately vague about her wealth and background, suggesting a desire to remain anonymous.
* Her connection to Lowood: Jane notices that Miss Poole seems to have a particular interest in the institution and knows more about it than one might expect.
Although never explicitly confirmed, these details strongly point to Miss Poole as the benefactress.