Very rarely (if ever) is a class the primary source of any information worth citing. New information or ideas almost always are disclosed somewhere else before they're discussed in a classroom. It is exceptionally unlikely that something worth citing was revealed for the very first time in a high school classroom. That information came from somewhere else, and it should be acknowledged in a proper fashion.
One should cite the most relevant reference, not the one most convenient for the writer. In my line of work, for example, this means not simply citing a recent example of someone running a particular type of reaction, but citing seminal papers and major reviews on that reaction.
If you notice this notice then you will notice that this notice is not worth noticing.