One big reason this shift isn't happening is that it creates coordination problems: for many families, everyone leaves for school at the same time, so if you move grade 9 forward an hour, you have to do the same with grade 4 to get any benefit; the 9th grader is taking the 4th grader to school. That problem stopped this shift dead recently where I live.
To answer some of the questions posed to this comment, I really think the problem is many school districts do not have enough buses and drivers to transport their entire student body at the same time.
By having secondary start first, you require 50% less buses.
If we were to switch secondary goes second and primary age students go first, your primary age children would leave for school around 7am and arrive home at 3pm. Most parents are not home at 3pm and this causes a large problem for families. In some instances, the older children who are in secondary schools -- watch the younger children until parents get home.
So we started really driving people to push notifications, but users liked the text messages. Especially if they believe your messages are important. As the gateways become more unreliable, users became frustrated and weren't interested in switching over to push notifications.
I did all the research you did, and while we were profitable taking on SMS was going to become a major cost. I couldn't do it. So we took these steps:
1. Immediately stopped providing SMS gateway texts to all new customers. This stopped the expectation that text messages were included. These customers were told we only had push notifications.
2. We went with Twilio and with our decade of data we had a good idea how many text messages per customer. We begin offering a new package that included text messages. We really didn't mark up the price at all, we just covered costs.
When a customer would complain about only have push notification option, we sold them the text message package. When the SMS gateways were unreliable for a customer, we sold them the text message package.
The product is still sold like this today.