The date is good for our school and other (international) schools who deliver the IB programme (rather than A levels), as our exam period is May-June, so although the CM winner is announced during our last week of term, at least the Shadowers (Y7-10) are still in school.
As for clashing with other co/curricular activities - please show me a SINGLE week in any large school's calendar where NO clashes occcur and I'll buy you a latte!