No, the grades of a school analogy is much too simplistic. What Baha'is believe might make what was taught in the earlier religions wrong, but not because whatever scriptures that had are wrong when they were revealed, but rather because they were misunderstood and/or mucked up by the followers of those religions since they were revealed.
From my perspective it is really a moot point what the older religions taught
Yes, I agree, the grade school analogy is too simplistic. Here's some basics of Buddhism...
Do Buddhists believe in God?
In a word, no. There is no divine creator god or supreme being in the Buddhist teachings, so that Buddhism is often called a nontheistic religion. The historical Buddha began as an ordinary person, who gained awakening by training his own mind and apprehending the true nature of reality. His enlightenment wasn’t bestowed through communion with a superior, external force but through his own efforts. And that’s a major point of the Buddhist story. From the Buddhist point of view, a personal god isn’t necessary: we each have the raw material to achieve our own liberation.
Creator in Buddhism?
Buddhism is a religion which does not include the belief in a
creator deity, or any eternal divine personal being.
[1][2][3] It teaches that there are divine beings or gods (see
devas and
Buddhist deities), heavens and rebirths in its doctrine of
saṃsāra (cyclical rebirth), but it considers none of these gods as a creator or as being eternal (they just have very long lives).
[4] In Buddhism, the devas are also trapped in the cycle of rebirth and are not necessarily virtuous. Thus while Buddhism includes multiple gods, its main focus is not on them.
Can anyone become enlightened?
Buddhism teaches that everyone has the capacity for awakening. By following the path of practice the Buddha laid out, we can all eventually free ourselves from suffering, no matter who we are.
That said, most schools of Buddhism also teach that we each achieve enlightenment according to our karma—the consequences of our thoughts and actions—and we may have a pile of negative karma that will take a lot of work to clean up. The more skillfully we work at following the path, the better. It may take a long time, but we’ll get there.
I think it is worth reading all that it says at each link, but what I quoted will do to make my points.
First, as we know Hinduism has multiple Gods. But it says, "a personal god isn't necessary", in Buddhism. So, does that include the "Abrahamic" God that the Baha'is believe in?
Next, Buddhism does not "include the belief in a creator deity". So, no, Buddhism doesn't believe in a deity like the creator deity of the Abrahamic religions.
Then, it says that "everyone has the capacity for awakening". So was Buddha special? Was he a manifestation? No, not if anyone can reach that same level of enlightenment. Plus, it takes many lifetimes of reincarnating to finally get to the level of being able to reach enlightenment.
So compared to any of the Abrahamic religions, what is all this? I don't see any thing that would make me think these ideas came from the same source as Judaism, Christianity, Islam or the Baha'i Faith. And this is just the basics. The only thing I see a Baha'i can do is to say this was all false. All based on misinterpretations of the "original" teachings, (that supposedly had a creator God) and were based on added traditions made up by the follows of the Buddha. Which my be true. All the Buddhist Scriptures could be made up by the Buddha's followers. But, again, Baha'is seem to support the Scriptures of all the past religions, while at the same time leaving loop holes by which they can deny anything they need to... or, as with the case of God, add in what they need to.
Then, Baha'is do the same thing with the prophecies. They leave out what they don't need and take what they can use to make Baha'u'llah Maitreya. But why are they trusted? When were they written and by whom? Yet, the Baha'is use them.