You’re right and this teacher needs to be relax. My wife and I try to say, “Oh my gosh!” around the house as a way to head off this exact situation, but thats just us being probably too cautious. (Also a likely reaction to a family member who has a sailor’s mouth and now her 3yo says ‘fuck’ so that’s neat)

Does your little guy watch Bluey? The parents/kids in that use a lot of funny, but kid-safe phrases that you could introduce. Plus, then you’re watching Bluey which is always a good choice.