- Sorting Hat - It is Harry's first experience at the school. He has no idea onof how Sorting works. His greatest fear is being sent back to the Dursley'sDursleys'.
- Midnight Duel - Harry is only in the second week of school and already breaking rules by arranging to duel underage student. I don't know about you, but my school expelled students who got into fights.
- Forbidden Forest - Objectively, a student has broken at least three separate rules, havehaving an illegal creature, and hashaving a magical item of unknown properties. Suspension is a valid concern in this instance!
- A Peck of Owls - Harry had "clearly broken" the International Statue of Secrecy. He had broken not a school rule, but a law. He would be expelled and not even be able to practice magic.
- Transfiguration Class - There is no threat of expulsion here, only not being able to learn Transfiguration. As this is a highly dangerous subject, this is not unreasonable.
- Quidditch - Hooch was exaggerating just a bit. She did not want any unsupervised flying attempts. One student was already injured, she did not want to risk another.
- Whomping Willow - Ron and Harry had stolen an unregistered magical item and most likely broken the International Statute of Secrecy. Upon apprivingarriving at school (as Snape makes clear) they crashed into the Whomping Willow and nearly destroyed it. Again, breaking laws and destroying property is really something to be concerned about!
- See above...
- Shrieking Shack - Breaking multiple school rules and AIDING AND ABETTING a known CRIMINAL! They are probably lucky Azkaban wasn't being mentioned!
- Sectumsempra - Harry nearly kills Draco. If Snape had not been there and recognized what had happened it is conceivable for Harry to be tried for murder. Expulsion and Azkaban would seem likely.
PART 2: Expulsion from Hogwarts is rarely enforced because it would essentially ostracize you from the entire Wizarding World. (Note I am not taking into consideration that Newt ScarmanderScamander was expelled, since FB was published later.)
The only expulsion we see from Hogwarts is Rubeus Hagrid. Once expelled, his wand was also snapped, because without completing his education the governing bodies have deemed it unsafe to practice untrained magic. Hagrid is also a special case because the reason for his expulsion is also related to the death of a student. Not being able to use a wand is going to severely limitslimit Hagrid's job opportunities. Dumbledore was gracious enough (and knew Hagrid was innocent) to employ Hagrid as the grounds keepergroundskeeper at Hogwarts.
In general, those who cannot perform magic are not treated very nicely within the magic community. Look at the two confirmed Squibs;Squibs: Filch and Mrs. Figg. Both are not thought of very highly by anyone, except again Dumbledore.
- Prior to OotP he is "The Boy Who Lived"! Let's be honest, teachers are going to give him the benefit of the doubt. Even more so since they realized he had no clue about what he was.
- OotP and beyond: Harry was indeed expelled over the summer after the Dementor Attack... However, even that could not stick as he still had the support of the majority of wizards (that and the charges were nonsense).
Recap: Expulsion is rarely used because of how detrimental it would be to the entire life of a Witch or Wizard.