For your son, put an undershirt on top of his regular shirt. If your daughter has undershirts, do the same for her. (The school won't let you put underpants on the outside though!) If they have some of those non-skid socks, let them put them on over their shoes once they get to school. Or put their shoes on the wrong feet if it's comfortable. (Boots might be easier.)
You might consider hair color - the spray stuff that washes out. If your kids are blond, send them in with black hair.
Or, have them dress as the opposite of what they are - kids dressed as adults. Have your son wear a tie and carry a briefcase instead of a backpack. If you don't have a briefcase, use a laptop case. Your daughter can do that too. Maybe someone has a couple of blazers you can borrow.
You could do spring stuff - have them wear one of those garden belts with little tools hanging off (trowel, etc.) and carry their stuff in a gardener's bucket. Or farmer attire - overalls, sun hat, etc.
But the easiest - and what most kids will do - is backwards and inside out.