No school bus

Sometimes this just happens, it is a glitch that I saw several times in my games. It also can happen that the car for your adult Sim will not show up.

If the resetsim cheat does not work, you will have to make them go to school "manually" by going to the "School" tab and clicking on the "Go to School" button:

This glitch usually goes away after there is a change in your Sim's life. Aging up from child to teen should do the trick (as does a promotion if this happens for an adult Sim with the car pool from work).

You could try to send the kid to boarding school and then calling it back from boarding school ahead of time. That should work too.