Interesting, I didn't know about Mr Dux! My first guess was the opposite: "to come dux in...", eg. "She came dux in English and Maths". Meaning she scored top marks in both those subjects. The dux is the top achiever or valedictorian, in academics or sports.
If someone told me "she duxed four of her subjects", I'd have to ask exactly what was meant: so did she fail, or gain top marks??