That isn't what the teacher said though. "just accept that these reactions happens and memorise them for the test. Don't try to understand it"

And in either case, there are far better ways to phrase that that aren't going to demotivate a kid to learn - "that's pretty advanced! We'll talk about that later in the year" or something along those lines.

More likely though, is that this teacher didn't know, didn't care, and couldn't stomach saying the words "I don't know; let's find out together" to a highschooler - which also would have been a far more productive answer!