Your husband was given a ticket with zero points because they don't know who the driver was, but he is still responsible as the owner of the plates. If they knew who the driver was, it comes with 6 demerit points. Since he received a zero point charge, they are not interested in proving who the driver was. All they need to prove is whether or not that plate passed a school bus while it was flashing. So if your husband plans on using the "it wasn't me" defense, it's not going to matter if it was him or not. The prosecutor isn't going to need to prove that.
They are going to have a trial where they will have to prove his plates passed the bus. Nothing more, nothing less. If they aren't able to do that, they aren't going to be able pursue this ticket any further.