Your Sim can get expelled for repeatedly skipping classes, failing to complete homework, and getting caught doing things such as pranking.
Pranking objects such as the whiteboard or speaker system can also get your Sim into trouble. Getting caught can result in detention and add to your Sim’s disciplinary record.
If your Sim continues to get into trouble at school, they will eventually receive three strikes. Once they reach the required number of strikes, they can be expelled from high school.
Expelled Sims receive the Expelled buff, which makes them Sad for several hours.
Getting expelled also prevents your Sim from attending university and can restrict access to certain careers.
Once expelled, your Sim cannot simply re-enroll in high school. They can eventually work toward earning a high school diploma online after becoming a Young Adult.
Keep in mind that simply having an F grade isn’t enough by itself to guarantee expulsion. The High School Years disciplinary system is what can ultimately lead to your Sim being kicked out.
If your Sim is being expelled despite behaving properly, you can use a mod such as The Sims 4: Don’t Get Expelled to prevent bugs from causing unwanted events.