There is no requirement for dependent to stay in school. The age is the only requirement.
Many years ago, the age limit was below 19 and for dependents over 19, they need to show that they are full time students and still depends on their parent.
BUT that has been changed and as long as you are under 22, at application (and you both were and you've got AOR), you are accept as dependent.
The only other requirement is that you have to be single (i.e. not married and not in a common law relationship - living with a partner for 12 months or more continously) .
So whoever "they" say about immigration officers calls school to confirm don't know what they are saying. It's possible but it's not to confirm that you HAVE to go to school. They could but it's to confirm that the information filled in is correct.
... as long as you do not fill the form to mislead them that you are still going to school, you are fine.
Nothing wrong to not go to school BUT you need to be trueful with the information you provided. (if you are working, fine, include work history. if you are just waiting and not going to school or work, just put that in. There is nothing wrong except that if you lie in the application, it's misrepresentation and that's very bad)
You have AOR, if you want to update any of the forms, you can submit updated form with letter of explianation and send a web form with the application number.