It depends on the bus so you should ask your school's transportation department.
But rest assured, if it is a proper school bus (and not a van or minibus style), then your child shouldn't need to wear a seat belt. I know this sounds weird, but it's true. Buses are built differently than cars and they take crash impact differently. Your child should actually be safer driving to school in a bus than a car. This is based on studies done by national transportation safety boards in the US (and other countries).
As an aside, if your child is still in a booster seat then lap only school "bus" is not appropriate. Using a booster seat with lap only restraint systems is dangerous and every single car seat manufacturer explicitly states that booster seats cannot be used w/ lap only belts. I've brought this up with a large nursery chain here that REQUIRES booster seats, but they have ignored me after multiple calls and a very detailed email where I sent them copies of the booster seat manuals. So please be aware of this.
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