Your science teacher was wrong, unfortunately. In Classical Latin, datum is pronounced as [ˈd̪ät̪ʊ̃ˑ] "dah-too(m)" and likewise data as [ˈd̪äːt̪ä] [ˈd̪ät̪ä] "dah-tah."
Not that Latin should really have a say in how we speak English anyhow.
More like [ˈd̪ät̪ä], no long vowel. There's also some disagreements if short /a/ was [ä] or [ɐ], given the symmetry with /e i o u/ as [ɛ ɪ ɔ ʊ]. (I can go deeper on this if anyone wants.)
Another thing that people don't often realise, when they say "you should pronounce it like in Latin!", is that Latin /d t/ were different from English/German /d t/. They were considerably less aspirated, and as your transcription shows they were dental.
That's just details though. Your core point (Latin didn't use a diphthong in this word) is 100% correct.
That's my B, I was looking at Ecclesiastical Latin for that one :3
Interesting points though, thanks for the elaboration. Shows the layers of silliness that is depending upon other languages for the way we pronounce words.