1). A child experiencing these feelings is not alone and can seek help from the school counselors and psychologist.
2). This is likely a common experience for school systems to have "witness" children approach them about not believing their parents religion and all the emotions that come with it. As a former elder, most of the children in my congregations "fell away" and with some, it was obvious they unburdened themselves to school authorities and got help. It would not be surprising if part of a teachers education were courses on "dealing with the Witness child".
3). The child should have them help develop a plan for independent living, tell them his/her feelings and that you need mentoring for a successful life.
4). Start your new life by doing the best you can in school now. This will be making progress for a new future and taking your mind off things until you get out. Get further education after high school.
5). Live a disciplined life and be successful, avoid drugs, alcohol and promiscuous sex. Avoid people who are not on the path to success. Be a good person. Nothing confirms to people "still in" that they have made the right decision than people who leave and then go on to live a trashy life. Don't challenge your parents or anyone else in the congregation about their beliefs. Writing a letter about all your feelings can be therapeutic, but sending it to anyone will have little effect. Dis-associating, making scenes or insisting on a "bad boy" boyfriend only re-enforces them into the belief the problem is that you let the "apostates" reach you or immorality is the root cause. The best way is to set yourself up as successful person and FADE AWAY. That is very discouraging to people still in. Then when you happen to see them, you can tell about how well you are doing.
6) I realize the poster of this thread is an adult, so was rather hard to frame this reply. However it is an opportunity for child readers.