So, it depends on what you mean by "realistic". If you mean, "was this the kind of treatment that most girls of the period could expect when they went to a boarding school," then no, it's not representative of reality (if only because girls' education was so much less institutionalized at the time). If you mean, "was this based on something in reality," then the answer is yes.

Jane Eyre is not an autobiographical novel, but it does contain some autobiographical detail. Lowood School, where Jane is sent by her aunt to get her out of the way and punish her, is based on Cowan Bridge School, est. 1823, a year before Charlotte and her sisters attended it. According to Elizabeth Gaskell's biography of Charlotte Bronte, the school was largely run by Rev. William Carus Wilson; the plan of the school was to give clergymen's children a thorough education at a bargain price, with the fees charged to parents being entirely to cover supplies and boarding, and the teachers paid through charitable donations. (Incomes for vicars and rectors were generally not large, but as technical members of the gentry they were expected to educate their children to a relatively high degree.)

Gaskell seems to have undertaken some level of personal research, including interviews with other women who'd attended and even one of the teachers, as she gives a very detailed account of it. She walks a very careful line in criticism of Wilson, who was a generally well-regarded figure, and was still alive when her biography was published. For the most part, she claims that the abuses described by Charlotte occurred because he didn't hire good people. For instance, Gaskell says that Wilson took care to ensure that the food ordered was the best quality, but that the cook was "careless, dirty, and wasteful".

... at Cowan's Bridge School, [oatmeal] was frequently sent up, not merely burnt, but with offensive fragments of other substances discoverable in it. The beef, that should have been carefully salted before it was dressed, had often become tainted from neglect; and girls, who were schoolfellows with the Brontes, during the reign of the cook of whom I am speaking, tell me that the house seemed to be pervaded, morning, noon, and night, by the odour of rancid fat that steamed out of the oven in which their food was prepared. There was the same carelessness in making the puddings; one of those ordered was rice boiled in water, and eaten with a sauce of treacle and sugar; but it was often uneatable, because the water had been taken out of the rain-tub, and was strongly impregnated with the dust lodging on the roof ...

Then she says that the teachers were told to only mind education, and that when a few of them did report how bad the food was, he assumed without inspecting the situation that they were "pampering the appetite" and allowing the children to focus on "carnal things". However, she was eventually replaced with a cook who provided much more sanitary and edible food.

Likewise, both the kind Miss Temple and loathsome Miss Scatcherd were based on actual teachers Wilson had hired; the woman who was the basis for Miss Scatcherd had once verbally and physically abused Charlotte's dying older sister Maria for not getting up and dressed fast enough. Gaskell believed Maria to be a pattern for Helen Burns, and the sickness that killed Helen was based on a fever that spread through the school, although not the one that killed Maria. (Another telling anecdote about Wilson: he brought a laundress back to the school in his gig to help him figure out what was going on, and when she told him the girls were sick and she needed to get home before she took sick as well and brought the contagion to her own children, he drove away and left her there to force her to nurse them.) Charlotte also had a friend a bit older than herself, called Mellany Hane, who was from the West Indies and protected Charlotte from bullies, not much like Helen in temperament.

Like Lowood's Mr. Brocklehurst, Wilson tended to lecture the girls about being humble and grateful for the charity bestowed upon them, thinking that it would actually make them humble rather than affronted. While Gaskell goes out of her way to say that he had many good qualities that simply weren't on view to Charlotte because of her position, it sounds like Brocklehurst isn't far off from the truth of how Wilson behaved toward the children.

That being said, A Vindication of the Clergy Daughters' School and of the Rev. W. Carus Wilson from the Remarks in The Life of Charlotte Brontë was published by a Henry Shepheard in 1857, shortly after Gaskell's biography, and Shepheard makes the fair point that the Brontes were only at the school in its earliest days (it continued to exist almost until the present day, in one form or another, and Wilson remained in charge until 1850), and that that wasn't a fair position to paint a picture of misery as representative of the school as a whole under Wilson. He also points out the difficulty of trusting anonymous accusations made through an intermediary or the memories of a young child twenty years later, and corrects errors from Gaskell's book and misrepresentations in Jane Eyre (there were 16 students to start with rather than 70-80, and only 53 by the time Charlotte left; Wilson's daughters were too young to be the fashionable belles that come to see Brocklehurst's school in the novel; no children at the school died of the fever, as happened in the novel, and Wilson wasn't made to come under the control of better directors). Shepheard could do with calming down his rhetoric a tad, but it sounds as though for once the truth lies in the middle.