In another question, you stated that you are extremely unfit at the moment. If you're overweight and unfit, it really wouldn't be fair to the rest of the team for you to play since you might not be able to keep up with the more fit players.
Your school could also be concerned about your health and not want you to over exert yourself while at your present weight.
If they let you practice with the team, and you eat healthy (as was suggested in that other question I mentioned), that 38 pounds should come off fairly easily. Also do a little weight training to help build muscle, which burns calories faster.