Actually any of the three sentences would be accepted as "correct" and fine in the USA :-) Teacher One is using "effective" in place of "starting" probably because it is much more formal in the language of business, although it indicates the same reality as "starting". Teacher Two uses the word "starting" but changes the order of the sentence, putting the information about timing first, "Starting next month"....then stating what will occur in that particular time. All three sentences are correct, but the way of Teacher Two is the best choice of the three ways, it flows more naturally and sounds more natural.

Thank you so much.

They're all pretty much correct but Teacher Two's sentence is probably the safest and most appropriate for formal composition.