224 units is not enough to build even an elementary school around

Why not? If the housing is meant for families, let's assume a modest 60 percent of the houses have families, and that they each have 2.3 children. That's a total of 309 children. My kids go to a school of about 350 kids. I understand that in some places they have huge schools with thousands of kids, but I really don't see the advantage of that. Smaller schools where everybody knows everybody have a lot of appeal.

Simple, an elementary school will have, at best, half of the school-aged children (ages 5 through 11). So based on your guess, of the 309 kids aged 0-18, only 100 or so will be at the elementary school. You might be able to make a small school out of that (it puts less than 20 in each grade) but more than likely, there is already a few elems in the area that can absorb the kids. And if not? Less than 20 kids/grade means a lot of attention.

To get at your guesstimate on how many kids are there to begin with, let's look at Marin county specifically: average household of 2.39 people (parents+kids) and 15% of those are between 5 and 18. So if you assume the units will be filled "on average" (which is prone to error but we are just guesstimating) you end up with 80 school aged kids. That's about 40 elementary aged kids.

TL;DR: Your guess is way high.