Your dream is rich with symbolism and reflects various themes that might be present in your waking life. Here are some insights into the elements of your dream:

1. High School Setting: Returning to high school often symbolizes a revisitation of past experiences, insecurities, or social dynamics that you may be grappling with in your current life. It could indicate feelings of vulnerability or a need to confront unresolved issues from that time.
2. Twitching and Infected People: The imagery of people twitching and showing their veins could represent anxiety or fear of losing control, either in yourself or in your surroundings. It might also reflect concerns about societal issues or feeling overwhelmed by external pressures.
3. The Syringe: The syringe filled with green liquid signifies an opportunity or a choice that feels risky or uncertain. It can represent a decision about how to confront a situation or an internal struggle regarding your own health or well-being.
4. The Cheerleader: The cheerleader turning into a threatening figure embodies a betrayal of trust or the loss of innocence. It may indicate that someone you once viewed positively has revealed a darker side, or it could symbolize your own fears about how others perceive you.
5. The Chase and Escape: The recurring theme of being chased and needing to escape suggests feelings of anxiety or pressure in your life. You may be feeling overwhelmed by responsibilities or facing a situation that you wish to avoid.
6. The Car and Police: Cars in dreams often represent your path in life and how in control you feel about your direction. The presence of the police could symbolize authority figures or rules that you feel are restrictive or harmful. Your instinct that they are not the "good guys" might reflect a distrust in authority or societal structures that you feel are failing you.
7. Night and Day Transitions: The shifting between night and day can symbolize fluctuation in your mood or mental state, representing confusion, fear, and clarity.

Overall, this dream suggests a strong sense of anxiety about control, trust, and navigating complex social dynamics. It might be helpful to reflect on any current stressors in your life that could be manifesting in these vivid scenarios. Consider what aspects of your past or present you may need to confront or reevaluate to find peace and clarity. Engaging in self-care practices or speaking to someone about your feelings could also provide relief from these anxieties.