Dreams like yours can be quite rich in symbolism and emotion. Here’s an interpretation of the key elements:

1. Relationship Dynamics: The presence of someone you have feelings for indicates a desire for connection and intimacy. The mixture of your current and old house suggests that you may be reflecting on past relationships and how they influence your current emotions.
2. Jealousy and Insecurity: The interaction with the girl who hands over cakes and the guy's comments about her could symbolize your fears of inadequacy or insecurity in the relationship. His reassurance might reflect your need for validation and trust, especially since you mentioned the relationship feels unstable.
3. Urgency and Warning: Your need to warn others in the dream could represent a feeling of responsibility or the need to protect those you care about. This might also point to anxieties about external circumstances affecting your life or relationships.
4. The School Setting: Returning to your secondary school often symbolizes revisiting past experiences or learning from them. The chaotic escape might reflect feelings of being overwhelmed in your current life or a need to confront unresolved issues.
5. Flood and Evacuation: The flood can symbolize overwhelming emotions or situations that you feel are out of your control. Your inability to swim suggests feelings of helplessness in navigating these emotions or challenges. This could indicate fears of being unable to cope with your relationships or other life challenges.
6. Falling: The act of falling in your dream may symbolize a loss of control or fear of failing in some aspect of your life. It can also represent a transition or a significant change you are experiencing.

Overall, your dream reflects a complex interplay of emotions related to relationships, insecurity, and the pressures of external circumstances. It might be helpful to explore these feelings in your waking life. Consider addressing your insecurities and communicating openly with those close to you. This could foster a sense of stability and connection that you're seeking.