Expelling kids for buying decorative swords on a school trip to England?  I have never been and will never be for "zero tolerance" policies, but that's not even what they're claiming!  They're claiming they used their "best judgment!"  
Here's a tip: if your best judgment finds you expelling kids completely for non-threatening behavior, you need to find a new judge.  You don't just throw kids away like that.  Suspend them, confiscate the swords, put them on probation for exhibiting poor judgment, but recognize that there's a difference between their behavior and an actual physical threat.  
Because if the consequences are all going to be the same, what reason do then have not to use them?*
Aside from the fact that they'd probably break.  Three swords for $60?  Definitely decorative.