Definitely in the context of schoolteacher - school pupil interaction, a sentence heard daily.
I haven't checked, but it might not actually be in that section. Spaced repetition learning systems such as Duolingo's usually put in items from earlier sections in order to reinforce the earlier learning. That is a routine part of vocab learning tools such as Memrise, Anki and others, for example.
You make a very good point; however, in this instance I'm not so sure it is the case. 'Cnoi' was introduced as a new single word within this topic before this sentence appeared. I don't remember it appearing before - but I could easily be wrong - so it would 'appear' to have been intentionally included within this topic.
This was my furst time through the topic. It doesn't hurt to have more 'random' things appear when repeating exercises, but it does seem a waste when sentances more relevant to the topic could have been used.
It would perhaps be relevant to 'Education', as mentioned by @rmcode above. Education appears (much) later as a topic, but I'm currently far from reaching that. So perhaps it was intended to appear there. Unless there was some big event in Wales relating to pupils chewing gum in the news at the time they compiled the course? :-)
You are right, it does get introduced in 'News'. No real reason, I suspect - it is just a handy word to know and it doesn't really belong in any particular topic. Perhaps it will find another home in Tree2...