When we've transitioned students with a similar profile from parent-driven or door-to-door special needs transportation to the "big bus", sometimes we'll have the student do one way only for a while (e.g., only the morning to school, or, more often, only the afternoon homeward), which gives them a chance to ease into the new routine, and also can relieve anxiety about having the whole day off on the wrong foot.
Another option is assigning a bus monitor/aide for the first few weeks, not so much to sit with them, but truly to keep an eye on how the bus ride actually unfolds, and to coach discreetly as necessary. Then, if successful, fade the monitor out.
But for some students, it's better to start the year as you mean to begin (i.e., on the bus from day one), as transitioning mid-stream can sometimes be worse than just starting off with a new routine.
Much depends on the child in question, and the bus milieu, including the specific bus driver.
