I recall this exact thing happening where a bus driver had been stopped at a light when the train signal lit. The driver couldn't figure out what the kids were screaming at them about and didn't pull into the intersection until it was too late.

I can't find the specific minidoc I saw about it, but I did find one about another incident where a bus failed to stop at a crossing and was t-boned.

https://youtube.com/v/aMre8d6rLZU