Dreams like yours can be rich with symbolism and meaning. Here are some interpretations based on the elements you've described:

1. High School Classmates: The presence of classmates, even if they're not your real life peers, could represent feelings of nostalgia or unresolved issues from your past. High school often symbolizes a formative period, and dreaming of classmates may indicate a desire to reconnect with certain aspects of yourself or unresolved dynamics from that time.
2. The Creepy Old Man: This character may symbolize feelings of being trapped or manipulated by authority figures or situations in your life. The old man could represent fears, anxieties, or negative influences that you feel are controlling or threatening your freedom.
3. Trapped in the House: Feeling trapped suggests a sense of helplessness or confinement in your waking life. This could relate to a situation where you feel your options are limited or that you are stuck in a negative environment.
4. The Sea and Escape: The sea often represents the unconscious mind or emotions. Wanting to escape through the sea might indicate a desire to break free from your current situation or to explore deeper emotional issues. However, the fear of being chased suggests anxiety about venturing into the unknown or confronting those feelings.
5. Your Sister as Effy Stonem: Effy's character represents someone dealing with significant inner turmoil. In your dream, she could symbolize a part of yourself that is struggling or is aware of deeper issues that need to be addressed. Her urging to leave the situation indicates a need for self-protection and a call to action.
6. Parents Befriending the Old Man: This could reflect feelings of abandonment or misunderstanding, especially from authority figures in your life. It may suggest that you feel unsupported in your concerns or that those who should protect you are instead siding with the source of your fear.

Advice: Consider what areas in your life make you feel trapped or anxious. Reflect on your relationships and whether there are dynamics that need addressing. It may be beneficial to communicate your feelings and concerns with those around you. Lastly, take time to explore your emotions and see if there are underlying issues that need resolution.