"Extended ASCII" is just a sloppy term for a bunch of other encodings that are not, in fact, ASCII.

If your BASIC class used (or emulated) a C64 or compatible, you were using https://en.wikipedia.org/wiki/PETSCII and if it used MS-DOC you were using https://en.wikipedia.org/wiki/Codepage_437

We used QBasic, but I don't recall what version (maybe 4.5?). Codepage 437 looks similar to what I recall seeing, though.