I was not a Christian when I went to school so I’m not sure how to go about it but maybe a start would be to get involved in a youth group through your church or another church in the area.
You said the Christians in school already have their lunch group but if they were true Christians they shouldn’t exclude you, that would be judging. Have you asked to sit with them? You didn’t mention. It’s okay to have non christian friends, even Jesus sat with sinners, just as long as you remain true to your faith and not fall into their habits if they aren’t so good.
A good way to witness at your age if you aren’t quite comfortable is to invite a friend to spend the night at your house on a Saturday and take them to Church with you on Sunday. That may open a door. I had my 10 year old daughter do that and one of the friends now goes regularly and loves it. I gave her a Bible for Easter. The Sunday school usually has programs and her mom attended and from there. The mom is now a regular and was even Baptized recently.
My husband teaches Sunday school for the 7th and 8th graders here and the kids have learned to respect him and look up to him because of the way he treats them. Most youth group leaders or the ones I have met through the Church really devote themselves to the youth and they all get along great in and out of the Church.
Hope this helps.