It's funny how bigots always turn to "free speech" to defend the indefensible. There is no such thing in the UK as free speech. There is no law that defends it. We have laws that curtail speech and actions in all sorts of ways. Hate speech, for example, is specifically outlawed.
Section 4 of the Public Order Act 1986 makes it an offence for a person to use “threatening, abusive or insulting words or behaviours that causes, or is likely to cause, another person harassment, alarm or distress”. Deliberate disrespect of someone's holy book is behaviour likely to cause another person distress, and in the right circumstances may also constitute racial harassment. These children obviously knew that making a Quran the subject of a dare would be very likley to cause distress to Muslims. They did not bring it in to read or to show their peers how respectful they could be. Yes, they are idiots, but being an idiots doesn't mean that actions are defensible. The school dealt with this in a measured and sensible way. Suspend and educate.
I am a Christian, but I have no similar attachment to the Bible, and would not personally get upset if it was subject to similar treatment. But I would expect a school to respond to the same actions with a Bible in a similar way because it is not the book, but the deliberately disrepectful behaviour that underlies the actions that is the issue.
I do wonder what the reaction here would be if a bunch of Muslims publically and deliberately set out to disrespect the Bible. Outrage I would imagine, and just as much from non-Christians I will bet. Because they would be attacking our "culture". "heritage" or "British values". Those lines are trotted out often enough, but only when it refers to doubling down on someone else - it's funny how many of those "British values" don't apply to the British.
It may be just a book to you, whether the Quran or the Bible - but it isn't to others, and basic human decency would argue that deliberate provocation designed to distress others in not the kind of behaviour we want in society.