Don't bother wasting your collective breath on this 'news item'.
The students' original school report is so shock full of basic errors, that it is completely worthless from a scientific point of view (yep, I can read Danish). In fact I am tempted to believe it was written by 9th graders without too much science experience...
The only reason this news came to prominence, is because some journalist had an axe to grind with someone or other, and did absolutely zero fact checking before running it in the national news in Denmark.
The students literally don't even seem to be aware that the 'radiation' they are so worried about, is radio waves, plain and simple. In fact, from their original photos my suspicion is that the two groups of plants were grown in two separate rooms, class rooms to be exact. If they were worried about the danger of prolonged exposure to the 'radiation' - which they directly state in their introduction they set out to prove is dangerous(!) - then it stands to reason the pupils may have attempted to vacate the 'danger room' as much as possible. Less students in room => less carbon dioxide exhaled => less growth.