There are two ways of doing this:-
1) Just change the Timetable - see: KB33911 - Nova T6 - School wants to begin new timetable for second part of the Summer Term
2) Run full promotion - see: KB70075 - EOY - Beginning the new Academic Year before September
Both KBs are on SupportNet.
A word of caution, changing the timetable could be in breach of the STPCD 2011 especially where Y11 teachers may lose gained time. Also changes to the timetable should be should be in consultation with staff and unions.
I know of two cases at the moment where the unions are considering action due to this.