"The Three-In-One" reveals the origin of the Magic School Bus....
...Joyti is trying to use herbal tea to think of something that fits the theme of "Three-In-One" for the "Gizmo That Goes" contest, Fiona stated that she and her big sister drink pots of the same herbal tea when they entered the "Gizmo That Goes" contest — also won it as well. Joyti asks what was the theme of the contest in Fiona's year. Before Fiona answered, the Magic School Bus itself hops up and down while honking happily — and then Fiona answered, "Magic." In short, the Frizzle twins built the Magic School Bus for the Magic-themed "Gizmos That Goes" contest. In the original series, Keesha was temporarily living with her grandmother after her parents' divorce.
This would explain the Series Continuity Error of Keesha having only her grandmother in the first series but two mothers in the second. Children of divorcing parents sometimes temporarily live with other relatives until the process is over. This was presumably the case with Keesha when her parents divorced after her mother came out as a lesbian. She probably stayed with her grandmother for a few months, but eventually went to live with her mother and new stepmother. One of Keesha's two mothers is a trans woman.
She used to be male and her dad, but came out as transgender. Keesha stayed with her grandmother (as with the first series) while her dad transitioned from male to female, and eventually went to live with her parents, which were now two mothers.