I understand that according to some "schools" at least historical ones that the historical Buddha didn't teach the Mayanaha.
Is that true?
At the time of the Buddha there were no schools, only one "type" of Buddhism was known: The teachings of the Buddha, the discourses and verses spoken by him.
Buddha taught the Dhamma-Vinaya, he did not teach Theravada, Mahayana or Zen.
After the Buddha passed away to PariNibbana, the monks got together in councils to unify and register the teachings of the master, they put together the suttas, the Vinaya, the verses (Dhammapada), Jakata stories etc.
After many decades there was a split in the Sangha due to different interpretations of the teachings and that was when this discussion of Mahayana and Theravada began. As Buddhism reached different countries, different versions of the religion appeared addapting Buddhism to the culture and older religions of the country. Tibetan Buddhism is a good example, it still has traces of Astrology and Shamanism in some of its schools (this is not criticism, I'm not claiming this or that school is right or wrong, I tried not to take any side on this answer and respect all views)
Yes, some schools say that, but according to Tibetan Buddhism the teachings of Mahayana were actually given by Buddha, during the second turning of the wheel, at the Vultures Peak. The emphasis is in the concept of emptiness and boddhichita or universal compassion. In the third turning of the wheel, there is not a specific place mentioned. They deal specially with the Buddha Nature that all sentient beings possess.
It's impossible to know what the Buddha taught, as even the earliest known writings were written long after he passed on. However, many schools claim they were the original words of the Buddha, either as a bid for credibility or as part of sectarian disputes.
Don't take any such claims seriously.