Without being able to observe your technique, it's extremely difficult to address the leakage problem. Do you have a private instructor, or could you ask your band director for help? What comes to mind is the possibility that your reed does not sit well on the mouthpiece, and that the leakage is between the reed and the rails. In that case, a new mouthpiece with level rails would help. If it's your mouth, then the only solution is increased practice--once you hear the leak, stop, rest, come back. Each time you should be able to go a little longer, developing the muscles to remain sealed for longer periods of time.