Being back in school symbolizes lessons in life, life's experiences, interrelationships, structure, competitiveness, group preferences, habits of behavior, and making choices.
A dream of being in school suggests that your reluctance to break old associations, or habits, is slowing your progress in your current life.
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Are you afraid of 'taking chances'?
Are you in a position for possible advancement at work or are enamoured of another man...and you're afraid to put yourself 'out there'?
Are you afraid to go after what you want for fear of failure, or rejection?
Do you feel inadequate, or unworthy, of a specific task, or relationship?
Do you worry that you're just not 'good enough' to do what you want to do?
Do you often have the "If only....." thought?

That you didn't have a relationship with the boy in school, and you are now dreaming about him suggests a feeling of regret, and it seems like your subconscious is telling you not to make the same mistake twice.
Just because we are out of school, doesn't mean we stop learning. There are always new things to learn, and likewise, new experiences to be had.
Get yourself out there and go for the brass ring! You never know what you're capable of until you try!

I hope this gives you something to go on....cheers!