Dream are symbolic expressions of your feelings about your waking life. Without knowing you and what was going on, it's hard to pinpoint the catalyst of your dream. However, the symbols in your dream prove good clues.
The school setting and classmates in the dream suggest that the dream is about something that is happening in your school life. If you are not in school, the setting can be symbolic of work. The gym, coaches and athletes may reflect your awareness of the competitiveness of the environment, whether it be school or work. Here are links to the definitions on this site of the key symbols in your dream:
dark:
dreamdoctor.com/dream-dictionary/definitions/dark.html
bathroom:
dreamdoctor.com/dream-dictionary/definitions/bathroom.html
hiding:
dreamdoctor.com/dream-dictionary/definitions/hiding.html
shot:
dreamdoctor.com/dream-dictionary/definitions/shot.html
death:
dreamdoctor.com/dream-dictionary/definitions/death.html
Try to identify where you feel under attack (shot at) in your waking life. The answer should reveal the identity of the "man" is in your dream who is taking shots at you (could he be a jock?)... probably about an issue that you feel is private, thus your hiding in the bathroom. Your dream reflects your feeling that whoever it is and for whatever reason he is doing it, the attack has hit home and has caused you emotional injury. The acceptance of death in your dream is dream shorthand for your sense that your life will not be the same... that "it's over"... dead... not literally, but symbolically. It's how you feel about it... which doesn't necessarily make it so.
The dream expresses your feeling that your life will change or transition as a result of the personal attack.
The main message of your dream... protect yourself emotionally.
Hope that helps!