You can still get a driver's license as an adult after dropping out of school but if you're still a teenager and you're not in school then you won't be able to get your drivers license or drivers permit.
When you're a teen your parents usually have to sign some paperwork along with you allowing you to get your drivers permit at first and then after passing the drivers test you get the drivers license.
And if you're a teen and dropped out of school you usually won't be able to get your drivers license so it's important to stay in school.
You also need to stay in school anyway to get your high school diploma because without the diploma you cannot get any jobs which you need to make an income to pay for living expenses.