Dreaming about an ex, especially one with whom you have had a complicated relationship, can be quite common and may reflect various underlying emotions or thoughts. Here are a few interpretations of your dream:

1. Processing Emotions: Your dream may indicate that you are processing the unresolved feelings you have regarding your past relationship. Even if you've moved on, aspects of that relationship could still linger in your subconscious.
2. Desire for Closure: The dream could symbolize a desire for closure or reconciliation, even if you consciously feel differently. Laughing and being at peace in the dream suggests that part of you wishes for a more harmonious outcome.
3. Reflection of Current Situations: Since you're about to start school and be on the same sports team, this dream may be a reflection of the anxieties or uncertainties you have about facing her in real life. Your subconscious might be trying to prepare you for the social dynamics that could unfold.
4. Similarities and Conflict: The fact that you were once similar and that you wanted the relationship to work might indicate that you are reflecting on the aspects of yourself that you see in her. This could be prompting you to think about your own identity and how it has been shaped by this conflict.
5. Healing Process: Dreaming of being at peace may suggest that you are in a healing phase, where you are learning to let go of past grievances. It might be your mind's way of showing that you are ready to move forward without the emotional baggage.

Consider how you feel about the dream and whether it brings up any new insights for you. It may be helpful to reflect on your emotions regarding this situation as you prepare to return to school.