How do I play a high note????

I am stuck on the high D. I can't go up any more- any attempt to play higher will just result in a high D, and I don't seem to be improving at all. The grade requirements for orchestra have to be grade 5 which means I have to play to at least a high F!! I have three months until the interview date is due. It's been more than 10 lessons and I'm still stuck on the D. Any tips and tricks would be GREATLY appreciated. Thanks fellow trumpeters.