9. If a student leaves before 18, their driver's license is suspended.
Keeping them in school until they can at least sign legal contracts and such is a good idea, I believe.
But once you get students who are 21 going to school with 15 year olds, I have only seen negatives. I teach in a large urban school (3200+ enrollment) where 85% of our students qualify for free or reduced lunches and 20% are special education.
I had a case two years ago where a 21 year old told me to fuck myself because I told him to put his phone up and pay attention. He stormed out of class. A few minutes later, one of our campus police officers, who are sworn officers and wear guns, saw him urinating on the side of the fieldhouse. The officer got close and told him to stop what he was doing. Student pulled a knife and gave the officer, a high school classmate of mine, a 3" deep stab in the abdomen.
He was caught 3 weeks later in Fort Worth, 300 miles from here.
5 years ago, an assistant principal gave me an 8th period (last period) class composed of 12 20 and 21 year old males. He told me not to issue books, teach lessons, or do anything but keep them in the room until the final bell of the day rang, because they were all drug dealers selling in the bathrooms on campus, and he wanted to make sure they were away from their market during school hours. He told me (in front of them) that if the building caught fire to leave, but lock them in and leave them. After that year, I told him never to ask me to do that again. It was just too demoralizing for me.
So for me, overage students need to go to our local junior college and get their GED, which they can do at no cost.
The good point of course is that we are trying to get even those who had some troubles and had some problems to still be able to get their education. But I think the GED option does that well.
I do believe there are more problems than advantages to the current in-till-22 system in Texas. Hope this helps.