Hi Oekoeloe
Unfortunately, I don't think there's away to stop the interruption. However, you can pause the game, click on the school bus icon when it shows up, to close out of it. This will stop it's action, mostly, see note below.
You may need to have your Sim continue whatever he/she was doing, by re-selecting it. Then manually make the Sim go to school when finished.
How to do this:
1.) Click the right arrow, by the moodlet icons, to show the Sim panels. (if you haven't already.)
2.) Click the School tab
3.) Click the Go to School button.
If by chance the school bus is still outside after your Sim has finished what he/she was doing, then click on the school bus and have them go to school.
Just to note: The bus will honk once or twice at around 8:30am, and will leave about 9am.
Hope this helps,
MrsFlynn