For writers there are two things that come to mind when I say this. The first is more literal. Take a workshop. There are many offered all over the area, and farther. One friend even attended one in Scotland! They are often run by groups of writers such as RWA (the romance writers association). Some are offered online. Teachers can include well known authors, editors, or your fellow local writer. These workshops cover all kinds of things. They can help you work on character, plot, setting, anything you could think of, in a welcoming place. You can listen to others talk about their problems with writing and get help with your own. Workshops are a great place to learn to be a better writer.
The second way to “go back to school” is to work on something on your own. Take a look at your writing. You will know what areas you have trouble with, whether that be a story element, or a type of grammar, or anything. Then practice. Try re-writing a section, or maybe giving a character a backstory. Anything you can find to help yourself.
Just because we are now adults does not mean that we can’t go “back to school.” Take advantage of the daily reminders and make your writing better.