If a chemical reaction is repeated multiple times, will it produce the same result?
The answer depends on whether you are looking at the level of a single atom or a large population of atoms. Because of quantum effects, there is a lot of randomness in the behavior of single atoms. If there are multiple possible outcomes, we can fairly accurately determine the probability of each outcome, but which outcome occurs is not predetermined.
With a large population of atoms (such as we have in any normal laboratory experiment like the ones your teacher is talking about), the large sample size means that we get the same result every time if we control every relevant variable (which is actually quite hard to do).