Dreaming about having a one-night stand with an old classmate, especially someone you didn't get along with, can reflect various underlying feelings or thoughts. Here are a few interpretations:

1. Reconciliation with the Past: This dream may indicate that you are processing unresolved feelings or conflicts from your school days. Engaging with this person in a dream could symbolize a desire to find closure or to come to terms with past experiences.
2. Exploration of Identity: The dream might represent your exploration of different aspects of yourself. The classmate may symbolize traits or qualities you associate with them, and the one-night stand could suggest a temporary exploration of those traits.
3. Desire for Change: Dreams like this can also hint at a desire for change or excitement in your life. If you’re feeling stagnant or constrained, the dream may reflect a yearning for new experiences or a break from routine.
4. Unresolved Emotions: It’s possible that there are lingering feelings about your past, whether it’s about relationships, friendships, or your own growth since then. This dream could be your subconscious mind's way of bringing those emotions to the surface.

To gain more insight, consider how you felt during the dream and upon waking. Reflecting on your current life circumstances and feelings about your past can help you understand the dream's significance further.