They didn't threaten him with arrest, they informed him that under the law he could quite possibly face arrest.
And his parents were present, kind of - the school is in loco parentis, and the article says his head of year accompanied him.
The article doesn't say it was anti-terrorist officers that interviewed him, just that they contacted the school - a regular officer interviewed him. Besides, it's not like 'anti-terrorist' officers would have interviewed him in full riot gear, that's just the department that deals with assemblies/marches.
Really nothing notable here, just a child informed that they may be breaking the law.