Check out the following passage from Massachusetts General Law Chapter 90 § 14:
When approaching a vehicle which displays a sign bearing the words “SCHOOL BUS” and which is equipped with front and rear alternating flashing red signal lamps which are flashing, as provided in section seven B, and which has been stopped to allow pupils to alight from or board the same, a person operating a motor vehicle or trackless trolley shall, except when approaching from the opposite direction on a divided highway, bring his vehicle or trackless trolley to a full stop before reaching said school bus and shall not thereafter proceed until the warning signals are deactivated, unless directed to the contrary by a police officer duly authorized to control the movement of traffic.
As you can see from the above statute, motor vehicles (as defined in § 1) have to stop for busses with the stop sign out, but bicycles do not.
So, unless you’re operating a trackless trolly, feel free to ride through those with impunity, knowing in your heart that the law is on your side. Just keep an eye out for pupils.