I agree in principle. However, taking "fun" out of the classroom puts the notion in our children's heads that learning and fun are mutually exclusive. At least in my opinion, that's a big mistake. Back in the day when I was growing up, my best teachers did everything in their power to make school and learning fun. I was a serious student, I learned a lot, and to this day continue to make use of much of what I learned because I still remember it. And the primary reason I still remember so much? Because I had fun, yes fun, learning. So instead of banning "fun" in the classroom, wouldn't a better alternative be to simply post a list of adjectives for "fun" in the classroom, and encourage the students to build their vocabularies by using these words, whenever possible, wherever they had previously used the word fun?
Viable alternatives that come to mind are: Enjoyable; Interesting; Entertaining; Lively; Exciting; Fascinating; Captivating; Engaging; and Amusing.
The students could even be encouraged to make up an acronym to aid their ability to quickly recall these words. "ACE LIFE", for example, could bring seven of these nine synonyms to the tip of any fifth grader's tongue in a heartbeat. Acing life (and school), is not so hard when fun is involved.