Here's the view looking north on 35 Ave SW. I snapped this shot from about half-way between SW Myrtle and Our Lady of Guadalupe. To get as close as I could to the driver's perspective, I stepped out into center of the right lane. When traffic was clear, of course.

Please note: I used the telephoto feature on my camera to enlarge the subject, so in real life the sign would NOT appear to be this big from where I was standing.
This was how things looked at about 1 PM today, Wednesday. As you know, it's been overcast, but the visibility is still pretty good. If the lights had been flashing, the sign would've been even more noticeable.
My conclusion:
The signage is visible from both directions on 35th. However, I can STILL see how this could sneak up on someone. There's just a few seconds (as the car flies) between when the "school zone" sign first becomes visible to drivers and when those drivers need to be going 15 mph slower.
Add to this the fact that there is another (older) sign just 15 feet or so behind the school zone sign, and the fact that this other sign also has flashing lights, and you can see how the new school zone sign might not register with some drivers who are not expecting it to be there.
Oveall I feel that SDOT has done a good job making the school zone signs visible, so I can't really fault them. However, given the special traffic circumstances on 35th, it might have helped if they'd put up "school zone ahead" signs or something.
On the other hand, perhaps that's coddling.
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One more thing: As you know, 35th is a very busy street and school kids almost never venture across it. In fact, the nearest controlled intersection where they do cross is Myrtle SW, which is actually outside the 20-mph zone.
However, the purpose of the slow zone is not to make it safer for kids to cross the street there. The purpose is to make it safer for kids to get into cars as they pull over in front of the school.