The problem: The school bus drivers do not know which students will be absent each day. In the PM, they are dropping off students whose parents may not be home. Solution: Keeping all the parties in real time connected loop. What platform would you use to solve the problem? An automated call center with dispatching or an App?.
who is the consumer of this information, the parents, the school system or the bus drivers? Keep in mind that schools won't adopt technology if a large percentage of their families won't be able to use it (i.e. smart phone apps) so maybe sms alerts would have greatest reach.
The solution is definitely developing a well-defined App. Call center are always an indirect way of connection between the driver/the responsible people and the parents. However, in an emergency or urgent situation, both sides want the most direct means to convey the information.
We recently developed a similar closed-circuit App to be used in some of the international schools in Asia, so I can help you to provide the initial guidance on UI/UX related issues.
It depends on the age of the students and the safety of the neighborhoods involved. For example, it would not be wise to have kindergartners handling smart phones.
If you could provide additional information, I could give you a more educated answer.