Dreaming of being in a classroom means you are experiencing the feeling of being a student. This can also mean that you are learning new lessons or that you want to start learning something new.
If you dream of being in a lower grade classroom with younger students than yourself, it can represent the feeling that you are unable to move forward in your life because you are being held back to repeat a grade.
If the classroom in the dream is full of strangers, it can signify your need for collaboration and socializing.