Early History
Hogwarts was founded over a thousand years ago by two wizards and two witches: Godric Gryffindor, Salazar Slytherin, Rowena Ravenclaw, and Helga Hufflepuff. Shortly after founding Hogwarts, Salazar Slytherin had a falling out with the other founders. Slytherin wanted to admit only pure-blood students, but the other three founders didn't agree. Slytherin left the school, but not before secretly building a secret chamber, which the other founders knew nothing about. When his own true heir ("the heir of Slytherin") returned to the school, they would be able to open the chamber, unleash a horrible monster that lived in there, and purge the school of all Muggle-born students.

Middle History
About 300 years after the school was founded, the Triwizard Tournament began between the three most prestigious magical schools in Europe: Hogwarts, Beauxbatons, and Durmstrang. This tournament was considered the best way for wizards of different nationalities to meet and make friends. The tournament countinued for six centuries, until the death toll became too high and the tournament was discontinued.

Recent History
The existence of the school was threatened twice when the Chamber of Secrets was opened. The first time it was opened was in 1942. Tom Riddle, the heir of Slytherin and future Lord Voldemort, opened the Chamber in his sixth year. When a girl named Myrtle was killed, the Ministry of Magic began talking about closing the school. As Riddle spent the summer in a Muggle orphanage, he didn't want the school closed, so he framed Hagrid and left behind a mysterious diary.

The second time was in 1992 when the Chamber was opened by Ginny Weasley under the influence of Riddle's diary. Ginny wasn't responsible for her actions: she was in a kind of trance. Lucius Malfoy had given her the diary, with the hope that she would be caught and take all the blame, thus bringing an end to Arthur Weasley's Muggle Protection Act. However, Harry Potter discovered the truth, the diary was destroyed and the monster living in the Chamber (a basilisk) was killed.

In 1994, the Triwizard Tournament was started again, though this time more safety measures were put in place. For example, no one under 17 years of age could compete in the tournament. However, Barty Crouch Jr, disguised as Professor Moody, entered Harry Potter in the Goblet of Fire, even though he was only 14 at the time, and used a Confundulus charm to trick the Goblet into forgetting that only three schools could compete in the tournament. Consequently, Harry became a fourth champion, to the great disgust of the representives for Beauxbatons and Durmstrang. Crouch Jr. made sure that Harry won the tournament and turned the Triwizard Cup into a Portkey, which carried Harry straight into the hands of Lord Voldemort. Harry escaped, but Voldemort rose again.