The Parents
Who: Your parents, your friends' parents, and especially the ones who show up: PTO or PTA members, booster clubs, and the parents who email the principal every week. Some are on the school board.
Power: They vote in school board elections, and administrators know which parents will call a board member. One organized parent group gets more attention than a hundred students acting alone.
What they care about: Their own kid, first. Beyond that it varies a lot: college admissions, academic rigor, safety, strict discipline or dress codes, religious values, and whether the school day fits their work schedule. Most are glad to hear from students, but don't assume they'll agree with you.
How to work with them: Start with your own. Explain the problem, show the evidence, and ask for one email or one meeting. Then ask the PTO president for five minutes at the next meeting. Frame your ask around what parents already want: "stop scheduling three tests on the same day" is an easy yes for anyone who's watched their kid melt down. If your idea changes the schedule, like a later start time, expect questions about drop-off and childcare, and bring answers (morning supervision in the library, bus times that still work) before anyone asks.