Label everything - twice!
They will tell you they did nothing, didn't play with anyone etc - this is not true but their brains are overloaded.
Ask them about school at bedtime - miraculously you'll get a 20 minute chat with detail as it buys them time 
They will be hungry when they come out.
They may have forgotten to wee and be bursting when they come out.
Have a 'nit' kit ready - they will get them and going to the late night chemist when you spot them at bath time is a PITA!
Turn their PE kit and uniform inside out and get them to turn it right way round and put it on - it will spend most of its life inside out and screwed up.
Check bag daily for letters - Primaries like to use a trees worth of paper a week for the eleventy billionth 'fun' thing to remember.
If they take packed lunch, use flip style sports bottle top drinking bottles. Screw on ones are never screwed on properly and wet bags are inevitable.
Buy PE pumps in all sizes higher too as they usually announce theirs are hurting them without warning and smugly pulling a new pair out of a drawer feels epic 
Be early for any assemblies or plays - queues form well in advance if times on tickets and front row equals good photos to bribe them with when they're 18.
Lastly enjoy it ... it goes by in a flash. DD starts Year 7 in Sept 