Decent professors distinguish between "using stuff we haven't taught yet" and "trivializing the assignment by using stuff that wasn't intended". If the assignment in an Algorithms course is to implement a square root function, calling "sqrt(x)" is obviously inappropriate. Structuring their solution using a do while() loop rather than while() loop is not.

I remember being marked down for using a ternary in Java for an introductory programming course at uni because the teaching assistant didn’t know what it was. Pretty minor loss of marks but it always annoyed me.

That said, having to do the introduction to programming class at uni despite programming for like a decade in high school was the bigger annoyance.

Same. I taught myself C when I was in high school, back in the early 90's. Fortunately, in college, I was able to test out of the intro classes for credit.