How to turn a lower-tier high school team into a good high school team

Step 0: Define what you mean by "good". If you want to be good as in winning your local league that has no good teams in it, follow Path 1 to Becoming Good. If you want to be good as in competitive in an area with nationally elite teams, follow Path 2 to Becoming Good.

Path 1 to Becoming Good

Step 1: Find and exploit the quirks of your local format.
Step 2: Practice a whole lot.

Path 2 to Becoming Good

Step 1: Practice a whole lot. In these practices, do not spend lots of time focusing on local formats, speed techniques, and one line questions. Instead, spend most of your time reading packets that have pyramidal, clue filled questions which expose the team to information they probably had not heard before and which will help them score more points in games against good teams.
Step 2: Attend tournaments where good teams show up and figure out what they do that makes them good (usually just practice). Also, attend tournaments using good questions and try to learn from mistakes, buzzes your team missed, and clues you didn't know, especially those other teams did.
Step 3: Practice a whole lot more. If you run out of High School questions, or if you want more exposure to hard clues, consider practicing on college questions.

Questions geared towards high schoolers that can help your team improve include NAQT's Invitational Series, HSNCT sets, PACE NSC sets, and numerous independently produced high school events like those found on the High School Quizbowl Packet Archive. College packets that can help your team improve include NAQT SCT sets, especially those produced for Division 2, ACF tournaments, especially Fall and Regionals, and the numerous independently produced tournaments found on the Collegiate Quizbowl Packet Archive. One strategy is to spend practice time focusing on the High School and novice Collegiate sets, given the high amounts of overlap between the top level of High School and the lower level of College, and then spend private time reading through the higher level college sets such as ACF Nationals to become more familiar with that level. This is especially useful if you are at a higher level of play than your teammates.