Three day suspension for sure. But some schools do 'in school' suspension. In my school, a student would be assigned to a small room next to the assistant principal's office. You're not locked in, but you have to stay there and ask someone in the office to accompany you to the bathroom. They wait for you outside. You are also escorted to lunch, buy your lunch, and they walk you back.
What do you do there? Your teachers are required to send assignments, classwork that could be done there and homework. I used to send TONS OF STUFF as I loved it when a pain-in-the-neck kid was sent there who seldom did any work. Out of intense boredom, most the work sent there would get done. I'd also send another student's notebook if there were notes missed or diagrams.
If the police were involved, however, an at-home suspension would occur and it's possible student and parent would have to meet with the superintendent before being re-admitting. Parents also are always required to meet with administration before the suspension is over. Some kids would sit in 'in house' suspension for longer than three days until their parents could get into school for the meeting.
If the student were deemed a continued threat, expulsion is a possibility. How did your fictional character get away with hitting a teacher? I had a student threaten me once and he was removed from public schools and had to attend a juvenile facility. I went straight to the police. If a student hits a teacher, that's assault and the teacher doesn't have to make the complaint. Administration can and usually will, even if the teacher doesn't want to out of fear or whatever the reason.
