I must first of all emphasize that I am not using the word queer as an insult, or in its old meaning of strange, but rather as an umbrella term for sexual and gender minorities that are not heterosexual or cisgender. (This is why I am using the phrase "magical humans" rather than "witches and wizards", as some magical humans do not identify as male or female. Though some centaurs and goblins are attending Hogwarts now, none of them are openly queer.) I would also like to state that few queer people at Hogwarts were open about it until fairly recently, and I do not like to speculate without proof.
Hogwarts was founded in 990 A.D. Homosexuality, cross-dressing, etc., were not officially banned only because they were illegal, as the founders themselves stated. Openly queer magical humans first began attending Hogwarts in 1981, when same-sex sexual activity was legalized in Scotland. Prompted by this, the Ministry of Magic issued official regulations permitting the attendance of all queer (not just gay or bisexual) magical humans. That year saw the attendance of the first openly gay (and indeed the first openly queer) student at Hogwarts, Melvin Mogarsky. And what a time he had of it! Whenever I hear of a first year feeling homesick, thinking they can't go on, I make sure to tell them Melvin's story-the beatings he endured, the spit and food in his hair, the month he spent as an anaconda thanks to Gregory Minchin's curse-and how he persevered, and said it was all worth it in the end.
Which is not to say that he accepted the horrid way he was treated, or that anyone else should accept such abuse. He reported it to the teachers (though I must sadly confess they did little to stop it) and fought back himself. The trouble was, there were so few who really understood him. Ravenclaw was the first house to admit openly queer magical humans in significant numbers, in the late 1980s, and it remained the singular house for openly queer, Jewish, and black magical humans until full integration was enforced by an act of Parliament, in 1992. It was not until 1993 that the first openly lesbian and bisexual students began to attend-Maria Dogtrotter and Samuel Egbert respectively. The first openly transgender student at Hogwarts was not out upon arrival but came out in 1995, whereupon she changed her name from Alexander Zaminsky to Zelda Gold. The first openly transgender male, by contrast, was out since before his arrival in 1998 (Martin Webb, formerly Julia Claxton.) The first openly non-binary student, Morgan Harper, also arrived as such in 1999, and the first openly asexual student, Jane Cotters, was also out long before her arrival in 2003.
It is a curious fact that the LGBT club should still call itself that. It was founded by Melvin, Maria, and Samuel in 1993, when LGBT was an increasingly popular acronym, and I suppose they thought that would cover everyone. Yet non-binary students and others who identify as queer but not LGBT have been pressing to change this name these past two years, and with no success yet; I suppose there is too much unsettled debate over what the club should be called. In any case, the club has been very successful, having secured a queer non-discrimination law covering all students and staff at Hogwarts as early as 1996.
One cannot conclude this history without mentioning Headmaster Albus Dumbledore. I spoke to him myself; it seems like a lifetime ago (certainly it was long before I became Headmistress) and he stated that although he had always been openly gay to his friends, family, and colleagues, he saw no reason to be so with the students, and feared it would lead to allegations of impropriety, particularly considering he became Headmaster in 1956, when homosexuality was little understood or accepted. I do hope, despite his excellent influence in other respects, Dumbledore's being in the closet in regards to the students will not be seen as a precedent that ought to be followed, just as his being closeted to all at Hogwarts during his own time as a student has not been.
Postscript: Those seeking further information may wish to know that the Muggle author J.K. Rowling, as well as writing of Albus Dumbledore's life in detail, has confirmed that he was gay, although she did not mention how open he was about the matter. She also confirmed the existence of an LGBT club at Hogwarts, though not any details beyond its existence. She has also written of centaurs and goblins, though none of those she wrote of are among our students.
Headmistress Phyllis Rosewood, Year 2015