Sounds like another case of over-kill to me too.
What problems *have* they actually experienced ?
What were ALL the causation factors ?
Are they all down to drivers of cars *other* than the parents of the kids ?
Can the school look at filtering children out in times stages (I know that can be an 'age' issue) or other exits ?
Can a park of the playground be used for pick-up's drop-offs ?
What ARE the real issues - really ?
Who complained & why ? What caused their complaint ?
Has any one been injured / killed ? When was the last ? And the stats for that road over the last 25 yrs ?
What if any recent / old changes have occurred to the road in the last 25yrs that may effect the road / drivers / pedestrians ?
What good can 20mph possible achieve anyway apart from everyone then being further distracted from the school and the children and the car activities etc etc !!! ?
Increases in accidents at 20mph will do nothing to make it safer in fact at least 4% more dangerous than a 30mph ....
If they really want to help show the dangers -- a proven Kent study showed that you take away the pavements and simply make drivers see that a person's risk was greater and they naturally slowed down - if they don't then helping the driver understand the risks, was, and is, far better.
Make sure that no un-necessary signs exist, in the nearby area, so that the existing school sign stands out !
.... and ... off the wall idea ...if they are really stuck and not much space, why not just build a portable road bridge (or two) to help youngsters cross the road ?

(A kind of twin (linking), large airport ladder ?) Off the all I know, but ... if you are going to spend a silly amount of money ....