It is probably not illegal. In most states, as long as everyone is either in a public or open place. Think about it, vacationers don't get arrested when they get pictures of strangers in the background. Or how about the last high school football game you were at, you could take pictures of yourself in the stands with your kids and their friends right?
It might be a different story if the bus is considered private property or quasi-public property. It is illegal to video tape in certain places considered private property without permission.
Furthermore it would be considered illegal in most states if it was done secretly and with the intent to "entrap." Because of the risk of entrapment, very seldom is video is admissible for use in court cases and is considered illegal in most, if not all states if it were done in this way (think investigative news story- reporters get in legal trouble often with hidden video stories and often get sued even if they uncover an injustice). Besides location of where it was taped, intent on how this video was taped or will be used would be a big determiner as to whether this is illegal.
For better understanding of the legal term entrapment, read up on something called "Sunshine" laws. The laws differ state to state. While they are mostly created to deal with media cases, they are often applied in cases involving private citizens.
There are some states where buses are equipped with on board video to prevent misbehavior on the part of students and the drivers. And in some of those states, the bus company/school district has the right to tape...but not necessarily the passengers on board if the bus is privately owned.
It's very complicated and you'd probably have to have an attorney in your area do some research into this.
This is why you will find video like this very seldom is permitted as evidence in a court case, yet winds up in the media on the evening news. People wanting to shed light on something will often get frustrated and will resort to giving the video to the media. But this is a risky thing to do...once again...the media can play hard ball once you turn the video over to them and call the shots. Sometimes in cases like that, the media will get burned and the court will issue a gag order so no one can see the video. Especially if this becomes a high profile matter that goes before a court of jurors.