Yep, 20mph for 300 meters outside a school makes perfect sense.
And I doubt that many folk would disagree.
Unfortunately, I'm sure you're correct about it being used as yet another way to fleece motorists.
On my regular sunny days run out there is exactly what you're describing.
i.e. a 30mph sigh as you enter a village. Almost immediately followed by a 20mph sigh for no apparent reason. Other than there being parked cars on both sides of the road.
Consequently, vehicles coming from both directions have to give way depending on which side the obstruction (parked car) is on.
Does that really req a 20mph limit, or just common sense & knowledge of the highway code?
The 20mph stretch is only approx 1/3rd of a mile long but I find it incredibly hard to stick to. As it's downhill in the direction I approach it from.
Obviously the safest way to ensure that you stick to a 20mph is to stay in 1st or 2nd gear. Which will mean increased revs. Which will mean more fuel is used & more pollution is pumped out of the exhaust. So how did the 20mph limit get past all the greenies?