Sprocket,

In PA, the answer to your question would be that you are required to stop any time that the school bus is stopped with red flashing lights.  The only circumstance where you are not required to stop is on a divided highway - that is, there is a median strip of some kind separating the lanes of opposing traffic.  No median strip, you need to stop. The middle turning lane that you describe is NOT considered a median strip - that is not considered a divided highway.