Dreams about being back in education are one of the most common people have.
It is generally about a lesson learned at that time that might apply to your life now and help you to find an answer to something that is applicable to your current circumstances.
It can also be due to a childhood or young adult memory that needs to be dealt with now in order for you to move on. Perhaps someone's behaviour in your waking life is reminding you of how a person acted towards you at that age.
More specifically, dreams about college are mostly about change and experimentation. If you are about to embark on a big change in your life or are in the midst of a shift now, it might remind you of the days when you were exploring new things at college.
Being in a classroom situation can refer to your desire to learn something new or that you might be in the middle of accumulating new knowledge now.
Another interpretation is that you need to move forward in your life. If you feel stagnant or frustrated with your current life either personally or professionally- it may be time to look to something new to present fresh challenges and help you accrue more knowledge.
School is very much like work in that you have an allocated amount of hours you must attend, you have relationships with your peers and you're always learning new things and producing work. If you have been thinking a lot about your career lately, this might be why it manifested itself as school as the two are very similar in many ways.