Quarrel caused when posh school fails to open (5)

I haven't cracked this one yet — but the brilliant team at Times for the Times have:

{h}ARROW (posh school) [fails to open]. My old school’s rich relative at the top of Harrow Hill.  ‘Quarrel / ARROW’ appeared in the QC on 30th May along with a MER over the terminology. There’s support for the definition in at least one of the usual sources but a quarrel is a square bolt as fired by a crossbow, so not what we normally think of as an arrow.