Have the kids ride in the bike lane on the road and have the sheriff enforce safe driving during the school commute. Put a deputy on a bike to escort them, with another in a car to nail any unsafe drivers.

Oops, I forgot that Poortola Valley can't afford any more sheriff's deputies. The other way to make the road safer would be to lower speeds: narrow the lanes, widen the bike lane, put in speed bumps and lower the speed limit to 25 where school kids are present.

Great to see this topic staying civil at this time.
My only thoughts are to open up the Firethorn trail segment of the Alpine rd dirt trail to bicycle riders. This trail was signed as "no bikes" a few years back,and is an old paved section of path that connects Los Trancos rd+Alpine. This would spread out riders(horse or bike) a bit more,and allow a road crossing away from fast turning vehicles at Alpine+Los Trancos rds., and allowing a safer Los Trancos road crossing for all trail users.
As far as spooked horses, I'm thinking the trail from Corte Madera Middle School(CMMS) to Alpine Hills CC ought to be made a bit wider, and overhanging brush+branches trimmed to allow for better visibility+less chance for a startled horse situation.
How about parents or the school providing bells for the bicyclists handlebars, ones that softly or loudly ring on their own as the pedaler rolls along. The horses can hear these bells from a great distance, and any competent Equestrian knows the "perk'd" ears that alert the rider that someone's comin their direction.
As a grandparent of a student at CMMS, I'm against any forced funneling of young cyclists onto Alpine rd, no matter the width of the road shoulder.
Knowing the Sheriff's office cannot provide 24/7 traffic control along that corridor.