Non-elementary school age children should ride regular transit buses.
The majority of the day, most roads are empty. The same is true of buses. But for a small portion of the day, both roads and buses are crowded. For buses, that part of the day is murder. Most UTA buses are single-door access During peak times, more people get on and off the bus. Thus, each stop takes longer, and thus the entire ride takes longer.
Apart from solutions suggested elsewhere, the idea of simply doubling capacity appeals--add more buses. After all, over half the people on buses are on the bus during those hours. Why not have more buses during those hours? Simply put, a lack of buses and drivers.
And thus my argument for putting teenagers on regular buses: Because the 'peak' of use for school trips and work trips don't line up. Schools trips peak in the afternoon, work trips peak in the evening. By using regular transit buses for teenagers, a transit agency would be able to support a larger number of buses, and thereby to increase service for work trips during the peak hour, making bus travel faster and more reliable.
It would allow school districts to do away with the enormous limited use bus fleets every school district is obliged to support, while making better use of the public public transit resource.
If fearful parents demand some buses be 'school children only', I am prepared to accept that compromise to increase the size of the available bus fleet for non-school children use for the rest of the day.