These are simply some brainstorming ideas and advice. Feel free to use, omit, or edit any of them to your preference (or you may want to ignore them entirely.)
If you’d like to continue with the school track…
Almost all, if not all, scenes in a story should develop some part of the story (plot, characters, world-building [this is mostly if the world has something novel that is not just the modern-day setting], etc.), likely multiple parts of the story at once. Thus, this introduction of the school and the character’s school life before her parents’ death should develop things possibly like this:
- Character development – show how the character is when she is carefree (she doesn’t have to be naive or anything, just without burden), untraumatized, and unaware of future events. This can give you a baseline to also show later in the story how she changes when she wants to take revenge and has experienced her parent’s death.
- Side characters – this can introduce side characters, suspicious people, friends, possible allies, or possibly the antagonist (depending on who you want the other characters to be and how much support you want the main character to have). If the school and the characters are not really important to the story, then side characters and classmates there can be more symbolic. This leads to the next point…
- Setup/buildup – you can use this period of time to emphasize one of two points (there may be others, but these are the most common): gut feelings/intuition or utter shock. More specifically, this event (which would be typically a monotonous day of routine) can help for contrast. The main character could either be experiencing a series of unfortunate events as school for some strange reason or have an extremely strange feeling in her gut all day for some reason. Even if the events were unrelated to the later event, such bad feelings or intuition can then help emphasize the distress of her parents’ death. On the other hand, you could also use this time to emphasize how normal her day was until this event. It was monotonous and possibly even fun at school, creating major contrast between how her day and life was before and after the event.
- Foreshadowing – this is very closely related to the point above. Foreshadowing can be used to emphasize her gut feelings or her unawareness. A student or side character could mock her parents or her for something related to her parents. The student could also ask a question about her parents in a more positive light because they were well-known businessmen or something. Have coincidental references to her parents can help emphasize both the close relationship the MC has with them (as she defends them or talks enthusiastically about them) which also helps emphasize the distraught she would have over losing them and can build suspense for the events that come later.
If you’d like to follow a different track…
Honestly, the points would be very similar to the ones above. Building suspense and developing the main character’s reactions to the later event tend to be some of the most important points of flashbacks (there are others, of course, but I find those are the most commonly developed). As for other options besides school:
- The parents know – they tell her not to go to school today and go somewhere else because they believe she may be tracked as she heads home from school. This adds foreshadowing and also shows how her parents are trying to protect her. This could also be used to develop a sense of guilt the MC may feel for not being able to do anything and “running away.” You would probably have to cover how her parents would get tied up in a mess that leads to their death later in the story, however.
- Coincidence – the MC has an accident or something on the way to school or heading back from it. Just covering this event will allow you to skip the school scenes and also help add the whole saved-by-coincidence from sharing the same fate as her parents because she isn’t present at the scene. The accident or whatever it is that stalls for time would have to be pretty time-consuming, however. You could use this time to develop the main character’s possible friendship or something, too, if you have them phone their friend to stay the night with them or something because of this event.
- Recklessness – if your main character was particularly reckless or was dragged into doing something reckless, this could also serve as a prior event. The MC could be doing something without her parents knowing that is somewhat dangerous or maybe even illegal. This would then emphasize her possible guilt when her parents die because she may think that she is responsible (because of her reckless actions or because she was doing something reckless that prevented her from being there when they needed her).
Some of these are very general, so I don’t know if they’ll be of much use. If there is anything that confuses you (because of my wording or something), feel free to ask for clarification. Anyway, I hope this helps.